cmd: extract cmd/go's cfg.LookPath into separate pathcache package
Lift out the LookPath cached lookup utility function into a separate
"cmd/internal/pathcache" package, so that it can be reused in other
commands in addition to cmd/go. No change in functionality.
Change-Id: Ica7fa627000843360c3e353d40a9a70605fbe033
Reviewed-on: https://go-review.googlesource.com/c/go/+/601479 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>