cmd/go: rebuild external test package dependencies

cmd/go: rebuild external test package dependencies

Was missing recompilation of packages imported only
by external test packages (package foo_test), primarily
because Root was not set, so those packages looked like
they were from a different Go tree, so they were not
recompiled if they already existed.

Also clean things up so that only one call to computeStale
is needed.

Fixes #3238.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5786048