go/types, types2: extract package name from test sources automatically

go/types, types2: extract package name from test sources automatically

This simplifies explicit tests and ensures that the error messages
contain the package name instead of a generic file name like "p.go".

Fixes #59736.

Change-Id: I1b42e30f53ba88456e92f990d80ca68ffc987e20
Reviewed-on: https://go-review.googlesource.com/c/go/+/486617
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>