cmd/go: use internal/goroot to check for a standard library package

Change-Id: I739728f976162a0b8425a93666e3694d967dceb7
Reviewed-on: https://go-review.googlesource.com/137436
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modload/build.go

@@ -14,6 +14,7 @@ import (
 	"cmd/go/internal/search"
 	"encoding/hex"
 	"fmt"
+	"internal/goroot"
 	"os"
 	"path/filepath"
 	"strings"
@@ -30,13 +31,11 @@ func isStandardImportPath(path string) bool {
 
 func findStandardImportPath(path string) string {
 	if search.IsStandardImportPath(path) {
-		dir := filepath.Join(cfg.GOROOT, "src", path)
-		if _, err := os.Stat(dir); err == nil {
-			return dir
+		if goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+			return filepath.Join(cfg.GOROOT, "src", path)
 		}
-		dir = filepath.Join(cfg.GOROOT, "src/vendor", path)
-		if _, err := os.Stat(dir); err == nil {
-			return dir
+		if goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, "vendor/"+path) {
+			return filepath.Join(cfg.GOROOT, "src/vendor", path)
 		}
 	}
 	return ""

src/cmd/go/internal/modload/import.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"fmt"
 	"go/build"
+	"internal/goroot"
 	"os"
 	"path/filepath"
 	"strings"
@@ -60,8 +61,8 @@ func Import(path string) (m module.Version, dir string, err error) {
 		if strings.HasPrefix(path, "golang_org/") {
 			return module.Version{}, filepath.Join(cfg.GOROOT, "src/vendor", path), nil
 		}
-		dir := filepath.Join(cfg.GOROOT, "src", path)
-		if _, err := os.Stat(dir); err == nil {
+		if goroot.IsStandardPackage(cfg.GOROOT, cfg.BuildContext.Compiler, path) {
+			dir := filepath.Join(cfg.GOROOT, "src", path)
 			return module.Version{}, dir, nil
 		}
 	}