cmd/go: fix module ... pattern to match standard library

The non-module ... pattern always has.

Fixes #26905.

Change-Id: I7b298747fb33b82c58d3e6a6bc6687b6e825e52c
Reviewed-on: https://go-review.googlesource.com/128997


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>