cmd/go: add Package.StaleReason for debugging with go list

cmd/go: add Package.StaleReason for debugging with go list

It comes up every few months that we can't understand why
the go command is rebuilding some package.
Add diagnostics so that the go command can explain itself
if asked.

For #2775, #3506, #12074.

Change-Id: I1c73b492589b49886bf31a8f9d05514adbd6ed70
Reviewed-on: https://go-review.googlesource.com/22432
Reviewed-by: Rob Pike <r@golang.org>