go/build: look in $GOROOT/src/cmd/foo/bar for import cmd/foo/bar

This lets us have non-main packages like cmd/internal or cmd/nm/internal/whatever.

The src/pkg migration (see golang.org/s/go14mainrepo) will allow this
as a natural side effect. The explicit change here just allows use of the
effect a little sooner.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/117630043

src/pkg/go/build/build.go

@@ -521,7 +521,12 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 
 		// Determine directory from import path.
 		if ctxt.GOROOT != "" {
-			dir := ctxt.joinPath(ctxt.GOROOT, "src", "pkg", path)
+			var dir string
+			if strings.HasPrefix(path, "cmd/") {
+				dir = ctxt.joinPath(ctxt.GOROOT, "src", path)
+			} else {
+				dir = ctxt.joinPath(ctxt.GOROOT, "src", "pkg", path)
+			}
 			isDir := ctxt.isDir(dir)
 			binaryOnly = !isDir && mode&AllowBinary != 0 && pkga != "" && ctxt.isFile(ctxt.joinPath(ctxt.GOROOT, pkga))
 			if isDir || binaryOnly {

src/pkg/go/build/build_test.go

@@ -193,3 +193,13 @@ func TestMatchFile(t *testing.T) {
 		}
 	}
 }
+
+func TestImportCmd(t *testing.T) {
+	p, err := Import("cmd/internal/objfile", "", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasSuffix(filepath.ToSlash(p.Dir), "src/cmd/internal/objfile") {
+		t.Fatalf("Import cmd/internal/objfile returned Dir=%q, want %q", filepath.ToSlash(p.Dir), ".../src/cmd/internal/objfile")
+	}
+}