[release-branch.go1.13] cmd/go: fix listing of ambiguous paths

Passing ambiguous patterns, ending in `.go`, to `go list` results in them
being interpreted as Go files despite potentially being package references.
This can then result in errors on other package references.

The parsing logic is modified to check for a locally present file
corresponding to any pattern ending in `.go`. If no such file is present
the pattern is considered to be a package reference.

We're also adding a variety of non-regression tests that fail with the
original parsing code but passes after applying the fix.

Updates #34653
Fixes #34694

Change-Id: I073871da0dfc5641a359643f95ac14608fdca09b
GitHub-Last-Rev: 5abc200103ffc122df05422d79cf30c3ba0ee646
GitHub-Pull-Request: golang/go#34663
Reviewed-on: https://go-review.googlesource.com/c/go/+/198459


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 33683f1d64df0cef2c598a84b741abb5af8abe5e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/198957


Reviewed-by: Jay Conrod <jayconrod@google.com>