cmd/go: properly ignore import comments for vendored packages rooted at GOPATH

Fixes #12232.

Change-Id: Ide3fb7f5fc5ae377ae8683fbb94fd0dc01480549
Reviewed-on: https://go-review.googlesource.com/13924Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/pkg.go

@@ -368,7 +368,8 @@ func loadImport(path, srcDir string, parent *Package, stk *importStack, importPo
 	if gobin != "" {
 		bp.BinDir = gobin
 	}
-	if err == nil && !isLocal && bp.ImportComment != "" && bp.ImportComment != path && (!go15VendorExperiment || !strings.Contains(path, "/vendor/")) {
+	if err == nil && !isLocal && bp.ImportComment != "" && bp.ImportComment != path &&
+		(!go15VendorExperiment || (!strings.Contains(path, "/vendor/") && !strings.HasPrefix(path, "vendor/"))) {
 		err = fmt.Errorf("code in directory %s expects import %q", bp.Dir, bp.ImportComment)
 	}
 	p.load(stk, bp, err)