go/types: print package path in error messages if package name is not unique

Change package qualification to print the full package path for packages
that have non-unique names (that is, where multiple different packages
have the same name). Use the package name as qualifier in all other cases
(but don't print any qualification if we're talking about the package
being type-checked).

This matches the behavior of the compiler.

Fixes #35895.

Change-Id: I33ab8e7adfae1378907c01e33cabda114f65887f
Reviewed-on: https://go-review.googlesource.com/c/go/+/209578


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>