cmd/go: don't let ... match GOROOT/src/cmd in module mode

GOROOT/src/cmd uses GOROOT/src/cmd/vendor, which module
mode simply cannot handle.

Exposed by making ... match the standard library, which it still should.

But for now it's fine to just exclude commands.

Change-Id: I2201b94445f11239022de8a2473aa3b573f405c0
Reviewed-on: https://go-review.googlesource.com/129055


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