cmd/compile: extract pkgqual from symfmt

The logic in symfmt for deciding how to package-qualify an identifier
is easily refactored into a separate function, loosely similar to
go/types.Qualifier's API.

Passes toolstash -cmp.

Updates #47087.

Change-Id: Ib3e7cc35a6577dc28df8eca79ba3457c48168e86
Reviewed-on: https://go-review.googlesource.com/c/go/+/333161


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>