Expanding interface method sets is handled during width calculation,
which can't be performed concurrently. Make sure that we eagerly
expand interfaces in the frontend when importing them, even if they're
not actually used by code, because we might need to generate a type
description of them.

Fixes #25055.

Change-Id: I6fd2756de2c7d5dbc33056f70b3028ca3aebab41
Reviewed-on: https://go-review.googlesource.com/122517
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>