cmd/go: address review comments

Address review comments from earlier CLs.
These are changes I was too scared to try to push
down into the original CLs (thanks, Git).

Change-Id: I0e428fad73d71bd2a7d08178cf2e856de3cef19f
Reviewed-on: https://go-review.googlesource.com/36257


Reviewed-by: David Crawshaw <crawshaw@golang.org>