cmd/go: add list -find to find packages but not resolve imports

cmd/go: add list -find to find packages but not resolve imports

This is needed by golang.org/x/tools/go/packages
and also gives a way to do a quicker scan for
packages with a given final path element:

	go list -find .../template

Change-Id: I092f4ac5ba7af7d727eb8204379fa436667061b9
Reviewed-on: https://go-review.googlesource.com/126716
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>