go/build: fix build

Presumably something about the very large go/build
doc comment breaks the build constraint parser in
cmd/dist.  I don't feel like debugging C code right now,
so move it into its own file.  If cmd/dist decides doc.go
is not part of the package, it will still build correctly.

R=golang-dev
TBR=golang-dev
CC=golang-dev
https://golang.org/cl/5722043

src/pkg/go/build/build.go

@@ -2,110 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package build gathers information about Go packages.
-//
-// Go Path
-//
-// The Go path is a list of directory trees containing Go source code.
-// It is consulted to resolve imports that cannot be found in the standard
-// Go tree.  The default path is the value of the GOPATH environment
-// variable, interpreted as a path list appropriate to the operating system
-// (on Unix, the variable is a colon-separated string;
-// on Windows, a semicolon-separated string;
-// on Plan 9, a list).
-//
-// Each directory listed in the Go path must have a prescribed structure:
-//
-// The src/ directory holds source code.  The path below 'src' determines
-// the import path or executable name.
-//
-// The pkg/ directory holds installed package objects.
-// As in the Go tree, each target operating system and
-// architecture pair has its own subdirectory of pkg
-// (pkg/GOOS_GOARCH).
-// 
-// If DIR is a directory listed in the Go path, a package with
-// source in DIR/src/foo/bar can be imported as "foo/bar" and
-// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
-// (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
-// 
-// The bin/ directory holds compiled commands.
-// Each command is named for its source directory, but only
-// using the final element, not the entire path.  That is, the
-// command with source in DIR/src/foo/quux is installed into
-// DIR/bin/quux, not DIR/bin/foo/quux.  The foo/ is stripped
-// so that you can add DIR/bin to your PATH to get at the
-// installed commands.
-// 
-// Here's an example directory layout:
-// 
-//	GOPATH=/home/user/gocode
-// 
-//	/home/user/gocode/
-//	    src/
-//	        foo/
-//	            bar/               (go code in package bar)
-//	                x.go
-//	            quux/              (go code in package main)
-//	                y.go
-//	    bin/
-//	        quux                   (installed command)
-//	    pkg/
-//	        linux_amd64/
-//	            foo/
-//	                bar.a          (installed package object)
-//
-// Build Constraints
-//
-// A build constraint is a line comment beginning with the directive +build
-// that lists the conditions under which a file should be included in the package.
-// Constraints may appear in any kind of source file (not just Go), but
-// they must be appear near the top of the file, preceded
-// only by blank lines and other line comments.
-//
-// A build constraint is evaluated as the OR of space-separated options;
-// each option evaluates as the AND of its comma-separated terms;
-// and each term is an alphanumeric word or, preceded by !, its negation.
-// That is, the build constraint:
-//
-//	// +build linux,386 darwin,!cgo
-//
-// corresponds to the boolean formula:
-//
-//	(linux AND 386) OR (darwin AND (NOT cgo))
-//
-// During a particular build, the following words are satisfied:
-//
-//	- the target operating system, as spelled by runtime.GOOS
-//	- the target architecture, as spelled by runtime.GOARCH
-//	- "cgo", if ctxt.CgoEnabled is true
-//	- any additional words listed in ctxt.BuildTags
-//
-// If a file's name, after stripping the extension and a possible _test suffix,
-// matches *_GOOS, *_GOARCH, or *_GOOS_GOARCH for any known operating
-// system and architecture values, then the file is considered to have an implicit
-// build constraint requiring those terms.
-//
-// To keep a file from being considered for the build:
-//
-//	// +build ignore
-//
-// (any other unsatisfied word will work as well, but ``ignore'' is conventional.)
-//
-// To build a file only when using cgo, and only on Linux and OS X:
-//
-//	// +build linux,cgo darwin,cgo
-//
-// Such a file is usually paired with another file implementing the
-// default functionality for other systems, which in this case would
-// carry the constraint:
-//
-//	// +build !linux !darwin !cgo
-//
-// Naming a file dns_windows.go will cause it to be included only when
-// building the package for Windows; similarly, math_386.s will be included
-// only when building the package for 32-bit x86.
-//
 package build
 
 import (

src/pkg/go/build/doc.go

+// Copyright 2011 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package build gathers information about Go packages.
+//
+// Go Path
+//
+// The Go path is a list of directory trees containing Go source code.
+// It is consulted to resolve imports that cannot be found in the standard
+// Go tree.  The default path is the value of the GOPATH environment
+// variable, interpreted as a path list appropriate to the operating system
+// (on Unix, the variable is a colon-separated string;
+// on Windows, a semicolon-separated string;
+// on Plan 9, a list).
+//
+// Each directory listed in the Go path must have a prescribed structure:
+//
+// The src/ directory holds source code.  The path below 'src' determines
+// the import path or executable name.
+//
+// The pkg/ directory holds installed package objects.
+// As in the Go tree, each target operating system and
+// architecture pair has its own subdirectory of pkg
+// (pkg/GOOS_GOARCH).
+// 
+// If DIR is a directory listed in the Go path, a package with
+// source in DIR/src/foo/bar can be imported as "foo/bar" and
+// has its compiled form installed to "DIR/pkg/GOOS_GOARCH/foo/bar.a"
+// (or, for gccgo, "DIR/pkg/gccgo/foo/libbar.a").
+// 
+// The bin/ directory holds compiled commands.
+// Each command is named for its source directory, but only
+// using the final element, not the entire path.  That is, the
+// command with source in DIR/src/foo/quux is installed into
+// DIR/bin/quux, not DIR/bin/foo/quux.  The foo/ is stripped
+// so that you can add DIR/bin to your PATH to get at the
+// installed commands.
+// 
+// Here's an example directory layout:
+// 
+//	GOPATH=/home/user/gocode
+// 
+//	/home/user/gocode/
+//	    src/
+//	        foo/
+//	            bar/               (go code in package bar)
+//	                x.go
+//	            quux/              (go code in package main)
+//	                y.go
+//	    bin/
+//	        quux                   (installed command)
+//	    pkg/
+//	        linux_amd64/
+//	            foo/
+//	                bar.a          (installed package object)
+//
+// Build Constraints
+//
+// A build constraint is a line comment beginning with the directive +build
+// that lists the conditions under which a file should be included in the package.
+// Constraints may appear in any kind of source file (not just Go), but
+// they must be appear near the top of the file, preceded
+// only by blank lines and other line comments.
+//
+// A build constraint is evaluated as the OR of space-separated options;
+// each option evaluates as the AND of its comma-separated terms;
+// and each term is an alphanumeric word or, preceded by !, its negation.
+// That is, the build constraint:
+//
+//	// +build linux,386 darwin,!cgo
+//
+// corresponds to the boolean formula:
+//
+//	(linux AND 386) OR (darwin AND (NOT cgo))
+//
+// During a particular build, the following words are satisfied:
+//
+//	- the target operating system, as spelled by runtime.GOOS
+//	- the target architecture, as spelled by runtime.GOARCH
+//	- "cgo", if ctxt.CgoEnabled is true
+//	- any additional words listed in ctxt.BuildTags
+//
+// If a file's name, after stripping the extension and a possible _test suffix,
+// matches *_GOOS, *_GOARCH, or *_GOOS_GOARCH for any known operating
+// system and architecture values, then the file is considered to have an implicit
+// build constraint requiring those terms.
+//
+// To keep a file from being considered for the build:
+//
+//	// +build ignore
+//
+// (any other unsatisfied word will work as well, but ``ignore'' is conventional.)
+//
+// To build a file only when using cgo, and only on Linux and OS X:
+//
+//	// +build linux,cgo darwin,cgo
+//
+// Such a file is usually paired with another file implementing the
+// default functionality for other systems, which in this case would
+// carry the constraint:
+//
+//	// +build !linux !darwin !cgo
+//
+// Naming a file dns_windows.go will cause it to be included only when
+// building the package for Windows; similarly, math_386.s will be included
+// only when building the package for 32-bit x86.
+//
+package build