[dev.typeparams] cmd/compile/internal/types2: fix a bug in package qualification logic

This is a partial port of https://golang.org/cl/330629, containing
only the actual bug fix and adjustements to another test file.

The respective test case has not been ported yet as it requires
some bigger adjustments.

For #46905

Change-Id: Ibd20658b8a31855da20cf56e24bcce9560656ca0
Reviewed-on: https://go-review.googlesource.com/c/go/+/336350


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