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 <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
Run-TryBot: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

nawawi/go,golang/go,nawawi/go,golang/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go,nawawi/go,golang/go