cmd/go/internal/par: add Queue as a simpler alternative to Work

par.Work performs two different tasks: deduplicating work (a task
which overlaps with par.Cache), and executing limited active work in
parallel. It also requires the caller to re-invoke Do whenever the
workqueue transititions from empty to non-empty.

The new par.Queue only performs the second of those two tasks, and
presents a simpler API: it starts and stops its own goroutines as
needed (indicating its idle state via a channel), rather than
expecting the caller to drive the transitions explicitly.

For #36460

Change-Id: I5c38657dda63ab55718497467d05d41744ff59f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/247766


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>