cmd/compile: make "imported and not used" errors deterministic

cmd/compile: make "imported and not used" errors deterministic

If there were more unused imports than
the maximum default number of errors to report,
the set of reported imports was non-deterministic.

Fix by accumulating and sorting them prior to output.

Fixes #20298

Change-Id: Ib3d5a15fd7dc40009523fcdc1b93ddc62a1b05f2
Reviewed-on: https://go-review.googlesource.com/42954
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>