[release-branch.go1.10] cmd/go: fix go list .Stale computation

If X depends on Y and X was installed but Y is only present in the cache
(as happens when you "go install X") then we should report X as up-to-date,
not as stale.

This applies whether X is a package or a main binary.

Fixes golang/go#25026
Fixes golang/go#25032

Change-Id: I26a0b375b1f7f7ac909cc0db68e92f4e04529208
Reviewed-on: https://go-review.googlesource.com/107957


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 9e0e6981)
Reviewed-on: https://go-review.googlesource.com/110076


Reviewed-by: Ian Lance Taylor <iant@golang.org>