cmd/go: do not turn list ./nonexist into a network lookup

If you're in a directory corresponding to x/y
and you run go list ./z, we do at some point
want to turn that into x/y/z. But if ./z does
not exist that will make the go command
check the network to see if it can find x/y/z.
That's clearly wrong: ./z means that directory,
nothing else. And it turns a typo into a long delay,
which is even worse.

Fixes #26874.

Change-Id: Iec15fa7b359af11b6a4fc6cb082e593658fb6e41
Reviewed-on: https://go-review.googlesource.com/129061


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>