go/types, types2: implement Info.PkgNameOf

go/types, types2: implement Info.PkgNameOf

For #62037.

Change-Id: I354f6417232708278d3f2b2d5ea41ff48e08d6b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/541575
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>