We'll always generate method expression wrappers for declared
interface types in their own package, so no need to generate them in
downstream packages.

Noticed by gri@ while looking into #21282.

Change-Id: I4fb7051b4e15297933da05fdd2b111d6b8f4178e
Reviewed-on: https://go-review.googlesource.com/106175
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>