cmd/go/internal/vgo: check src/vendor for "standard" imports too

Fixes vgo build net/http:

$ vgo build net/http
build golang_org/x/crypto/cryptobyte/asn1: cannot find module for path golang_org/x/crypto/cryptobyte/asn1
$

Change-Id: I1c27c31c67212a579091831b9f0dc534f498b969
Reviewed-on: https://go-review.googlesource.com/109336
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ross Light <light@google.com>

vendor/cmd/go/internal/vgo/build.go

diff --git a/vendor/cmd/go/internal/vgo/build.go b/vendor/cmd/go/internal/vgo/build.go
index f2c8d72..eeb5b7a 100644
--- a/vendor/cmd/go/internal/vgo/build.go
+++ b/vendor/cmd/go/internal/vgo/build.go
@@ -24,8 +24,10 @@
 
 func isStandardImportPath(path string) bool {
 	if search.IsStandardImportPath(path) {
-		dir := filepath.Join(cfg.GOROOT, "src", path)
-		if _, err := os.Stat(dir); err == nil {
+		if _, err := os.Stat(filepath.Join(cfg.GOROOT, "src", path)); err == nil {
+			return true
+		}
+		if _, err := os.Stat(filepath.Join(cfg.GOROOT, "src/vendor", path)); err == nil {
 			return true
 		}
 	}