go/types: use correct (file) scopes when computing interface method sets

This was already partially fixed by commit 99843e22
(https://go-review.googlesource.com/c/go/+/96376); but
we missed a couple of places where we also need to
propagate the scope.

Fixes #25008.

Change-Id: I041fa74d1f6d3b5a8edb922efa126ff1dacd7900
Reviewed-on: https://go-review.googlesource.com/109139


Reviewed-by: Alan Donovan <adonovan@google.com>