Ensure that compiler error suggestions after case insensitive
field lookups don't mistakenly reported unexported fields if
those fields aren't in the local package being processed.

Fixes #25727

Change-Id: Icae84388c2a82c8cb539f3d43ad348f50a644caa
Reviewed-on: https://go-review.googlesource.com/117755
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>