[release-branch.go1.11] cmd/go: fix modload response for std-vendored packages

This fixes a failure when using Go 1.11 to build App Engine code.

Change-Id: I008e8cf5ad4c568676d904deddff031a166f2d5d
Reviewed-on: https://go-review.googlesource.com/130138


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit c652a1b9)
Reviewed-on: https://go-review.googlesource.com/130616


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>