[dev.typeparams] cmd/compile/internal/types2: report unused packages in source order

1) Rather than map-iterate through all file scopes and collect unused
   packages, collect all imports in the Checker.imports list so that
   errors are reported in source order.

2) From cmd/compile, borrow the idea of a "dotImportRefs" map to map
   dot-imported objects to the package they were dot-imported through
   (we call the map "dotImportMap").

3) From cmd/compile, borrow the "pkgnotused" function
   (called Checker.errorUnusedPkg in this code) and clean up
   unused package error reporting.

4) Adjust unused package error message to match compiler message exactly.

5) Enable one more excluded test case in test/run.go.

Change-Id: I4e4e55512a6043a7fd54f576c7441e3dd4077d6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/287072


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