cmd/go/internal: remove some users of par.Work

par.Work is used in a number of places as a parallel
work queue. This change replaces it with goroutines
and channels in a number of simpler places where it's
used.

Change-Id: I0620eda46ec7b2c0599a8b9361639af7bb73a05a
Reviewed-on: https://go-review.googlesource.com/c/go/+/248326


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>