If a file in an ad-hoc package doesn't exist, go list should exit 0 and
return an dummy package with an error set on it. Since it doesn't do that
yet, add a work-around.
Updates golang/go#29280
Change-Id: I6019f28ce4770582f274919d1aa35d85a634687e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/171018
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>