cmd/go/internal/vgo: do not shadow GOPATH dir var

Allows actually detecting GOPATH paths without a package
import comment. Previously it may have appeared to work by detecting
github path.

Fixes golang/go#25176

Change-Id: I9b96e2b6c6c776a54d4fb1a996dabc02f92bd8ac
Reviewed-on: https://go-review.googlesource.com/111264
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

vendor/cmd/go/internal/vgo/init.go

diff --git a/vendor/cmd/go/internal/vgo/init.go b/vendor/cmd/go/internal/vgo/init.go
index 4a9b5bf..2c8b41a 100644
--- a/vendor/cmd/go/internal/vgo/init.go
+++ b/vendor/cmd/go/internal/vgo/init.go
@@ -242,8 +242,8 @@
 
 // Exported only for testing.
 func FindModulePath(dir string) (string, error) {
-	for _, dir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
-		src := filepath.Join(dir, "src") + string(filepath.Separator)
+	for _, gpdir := range filepath.SplitList(cfg.BuildContext.GOPATH) {
+		src := filepath.Join(gpdir, "src") + string(filepath.Separator)
 		if strings.HasPrefix(dir, src) {
 			return filepath.ToSlash(dir[len(src):]), nil
 		}