cmd/go: include external test files in the files sent to gofmt, govet, and gofix

Also, add XTestGoFiles to the go command's public api.

Fixes #2649.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5502102

src/cmd/go/list.go

@@ -36,12 +36,13 @@ being passed to the template is:
         Stale      bool   // would 'go install' do anything for this package?
 
         // Source files
-        GoFiles     []string // .go source files (excluding CgoFiles and TestGoFiles)
-        TestGoFiles []string // _test.go source files
-        CFiles      []string // .c source files
-        HFiles      []string // .h source files
-        SFiles      []string // .s source files
-        CgoFiles    []string // .go sources files that import "C"
+        GoFiles      []string // .go source files (excluding CgoFiles, TestGoFiles, and XTestGoFiles)
+        TestGoFiles  []string // _test.go source files internal to the package they are testing
+        XTestGoFiles []string // _test.go source files external to the package they are testing
+        CFiles       []string // .c source files
+        HFiles       []string // .h source files
+        SFiles       []string // .s source files
+        CgoFiles     []string // .go sources files that import "C"
 
         // Dependency information
         Imports []string // import paths used by this package

src/cmd/go/pkg.go

@@ -29,14 +29,15 @@ type Package struct {
 	Stale      bool   `json:",omitempty"` // would 'go install' do anything for this package?
 
 	// Source files
-	GoFiles     []string // .go source files (excluding CgoFiles and TestGoFiles)
-	TestGoFiles []string `json:",omitempty"` // _test.go source files
-	CFiles      []string `json:",omitempty"` // .c source files
-	HFiles      []string `json:",omitempty"` // .h source files
-	SFiles      []string `json:",omitempty"` // .s source files
-	CgoFiles    []string `json:",omitempty"` // .go sources files that import "C"
-	CgoCFLAGS   []string `json:",omitempty"` // cgo: flags for C compiler
-	CgoLDFLAGS  []string `json:",omitempty"` // cgo: flags for linker
+	GoFiles      []string // .go source files (excluding CgoFiles, TestGoFiles and XTestGoFiles)
+	TestGoFiles  []string `json:",omitempty"` // _test.go source files internal to the package they are testing
+	XTestGoFiles []string `json:",omitempty"` //_test.go source files external to the package they are testing
+	CFiles       []string `json:",omitempty"` // .c source files
+	HFiles       []string `json:",omitempty"` // .h source files
+	SFiles       []string `json:",omitempty"` // .s source files
+	CgoFiles     []string `json:",omitempty"` // .go sources files that import "C"
+	CgoCFLAGS    []string `json:",omitempty"` // cgo: flags for C compiler
+	CgoLDFLAGS   []string `json:",omitempty"` // cgo: flags for linker
 
 	// Dependency information
 	Imports []string `json:",omitempty"` // import paths used by this package
@@ -47,7 +48,7 @@ type Package struct {
 	pkgdir  string
 	info    *build.DirInfo
 	imports []*Package
-	gofiles []string // GoFiles+CgoFiles+TestGoFiles files, absolute paths
+	gofiles []string // GoFiles+CgoFiles+TestGoFiles+XTestGoFiles files, absolute paths
 	target  string   // installed file for this package (may be executable)
 	fake    bool     // synthesized package
 }
@@ -127,23 +128,24 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 	}
 
 	p := &Package{
-		Name:        info.Package,
-		Doc:         doc.CommentText(info.PackageComment),
-		ImportPath:  importPath,
-		Dir:         dir,
-		Imports:     info.Imports,
-		GoFiles:     info.GoFiles,
-		TestGoFiles: info.TestGoFiles,
-		CFiles:      info.CFiles,
-		HFiles:      info.HFiles,
-		SFiles:      info.SFiles,
-		CgoFiles:    info.CgoFiles,
-		CgoCFLAGS:   info.CgoCFLAGS,
-		CgoLDFLAGS:  info.CgoLDFLAGS,
-		Standard:    t.Goroot && !strings.Contains(importPath, "."),
-		target:      target,
-		t:           t,
-		info:        info,
+		Name:         info.Package,
+		Doc:          doc.CommentText(info.PackageComment),
+		ImportPath:   importPath,
+		Dir:          dir,
+		Imports:      info.Imports,
+		GoFiles:      info.GoFiles,
+		TestGoFiles:  info.TestGoFiles,
+		XTestGoFiles: info.XTestGoFiles,
+		CFiles:       info.CFiles,
+		HFiles:       info.HFiles,
+		SFiles:       info.SFiles,
+		CgoFiles:     info.CgoFiles,
+		CgoCFLAGS:    info.CgoCFLAGS,
+		CgoLDFLAGS:   info.CgoLDFLAGS,
+		Standard:     t.Goroot && !strings.Contains(importPath, "."),
+		target:       target,
+		t:            t,
+		info:         info,
 	}
 
 	var built time.Time
@@ -162,6 +164,9 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 	for _, f := range info.TestGoFiles {
 		p.gofiles = append(p.gofiles, filepath.Join(dir, f))
 	}
+	for _, f := range info.XTestGoFiles {
+		p.gofiles = append(p.gofiles, filepath.Join(dir, f))
+	}
 
 	sort.Strings(p.gofiles)