cmd/go: parallelize matchPackages work in each module

cmd/go: parallelize matchPackages work in each module

In each module matchPackages looks in, when doing the walk, do the
scanDir call in a par.Queue so all the read work can be done in
parallel.

Change-Id: I27153dbb3a2ed417ca24972f47134e9e914a55d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/404097
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>