go/types: fix a bug in package qualification logic

CL 313035 had a bug, initializing pkgPathMap by walking the imported
package being considered rather than check.pkg.

Fix this, and enhance our tests to exercise this bug as well as other
edge cases.

Also fix error assertions in issues.src to not use quotation marks
inside the error regexp. The check tests only matched the error regexp
up to the first quotation mark.

Fixes #46905

Change-Id: I6aa8eae4bec6495006a5c03fc063db0d66b44cd6
Reviewed-on: https://go-review.googlesource.com/c/go/+/330629


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