go/build: give better explanation for "cannot find package"

Fixes #4079.

Some example output:

% go install foo/bar
can't load package: package foo/bar: cannot find package "foo/bar" in any of:
        /home/dfc/go/src/pkg/foo/bar (from $GOROOT)
        /home/dfc/src/foo/bar (from $GOPATH)
        /home/dfc/src2/src/foo/bar

% GOPATH= go install foo/bar
can't load package: package foo/bar: cannot find package "foo/bar" in any of:
	/home/dfc/go/src/pkg/foo/bar (from $GOROOT)
	($GOPATH not set)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6899057

src/cmd/go/test.bash

@@ -150,6 +150,37 @@ if ! ./testgo list std | cmp -s test_std.list - ; then
 fi
 rm -f test_std.list
 
+# issue 4096. Validate the output of unsucessful go install foo/quxx 
+if [ $(./testgo install 'foo/quxx' 2>&1 | grep -c 'cannot find package "foo/quxx" in any of') -ne 1 ] ; then
+	echo 'go install foo/quxx expected error: .*cannot find package "foo/quxx" in any of'
+	ok=false
+fi 
+# test GOROOT search failure is reported
+if [ $(./testgo install 'foo/quxx' 2>&1 | egrep -c 'foo/quxx \(from \$GOROOT\)$') -ne 1 ] ; then
+        echo 'go install foo/quxx expected error: .*foo/quxx (from $GOROOT)'
+        ok=false
+fi
+# test multiple GOPATH entries are reported separately
+if [ $(GOPATH=$(pwd)/testdata/a:$(pwd)/testdata/b ./testgo install 'foo/quxx' 2>&1 | egrep -c 'testdata/./src/foo/quxx') -ne 2 ] ; then
+        echo 'go install foo/quxx expected error: .*testdata/a/src/foo/quxx (from $GOPATH)\n.*testdata/b/src/foo/quxx'
+        ok=false
+fi
+# test (from $GOPATH) annotation is reported for the first GOPATH entry
+if [ $(GOPATH=$(pwd)/testdata/a:$(pwd)/testdata/b ./testgo install 'foo/quxx' 2>&1 | egrep -c 'testdata/a/src/foo/quxx \(from \$GOPATH\)$') -ne 1 ] ; then
+        echo 'go install foo/quxx expected error: .*testdata/a/src/foo/quxx (from $GOPATH)'
+        ok=false
+fi
+# but not on the second
+if [ $(GOPATH=$(pwd)/testdata/a:$(pwd)/testdata/b ./testgo install 'foo/quxx' 2>&1 | egrep -c 'testdata/b/src/foo/quxx$') -ne 1 ] ; then
+        echo 'go install foo/quxx expected error: .*testdata/b/src/foo/quxx'
+        ok=false
+fi
+# test missing GOPATH is reported
+if [ $(GOPATH= ./testgo install 'foo/quxx' 2>&1 | egrep -c '\(\$GOPATH not set\)$') -ne 1 ] ; then
+        echo 'go install foo/quxx expected error: ($GOPATH not set)'
+        ok=false
+fi
+
 # clean up
 rm -rf testdata/bin testdata/bin1
 rm -f testgo

src/pkg/go/build/build.go

@@ -424,6 +424,13 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 		if strings.HasPrefix(path, "/") {
 			return p, fmt.Errorf("import %q: cannot import absolute path", path)
 		}
+
+		// tried records the location of unsucsessful package lookups
+		var tried struct {
+			goroot string
+			gopath []string
+		}
+
 		// Determine directory from import path.
 		if ctxt.GOROOT != "" {
 			dir := ctxt.joinPath(ctxt.GOROOT, "src", "pkg", path)
@@ -435,6 +442,7 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				p.Root = ctxt.GOROOT
 				goto Found
 			}
+			tried.goroot = dir
 		}
 		for _, root := range ctxt.gopath() {
 			dir := ctxt.joinPath(root, "src", path)
@@ -445,8 +453,28 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				p.Root = root
 				goto Found
 			}
+			tried.gopath = append(tried.gopath, dir)
+		}
+
+		// package was not found
+		var paths []string
+		if tried.goroot != "" {
+			paths = append(paths, fmt.Sprintf("\t%s (from $GOROOT)", tried.goroot))
+		} else {
+			paths = append(paths, "\t($GOROOT not set)")
+		}
+		var i int
+		var format = "\t%s (from $GOPATH)"
+		for ; i < len(tried.gopath); i++ {
+			if i > 0 {
+				format = "\t%s"
+			}
+			paths = append(paths, fmt.Sprintf(format, tried.gopath[i]))
+		}
+		if i == 0 {
+			paths = append(paths, "\t($GOPATH not set)")
 		}
-		return p, fmt.Errorf("import %q: cannot find package", path)
+		return p, fmt.Errorf("cannot find package %q in any of:\n%s", path, strings.Join(paths, "\n"))
 	}
 
 Found: