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 #24558.
Fixes #23818.

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>