go/types, types2: remove special case for external types in validType

Because validType doesn't modify global state anymore, there's
no need to ignore imported types. When we start tracking type
parameters, we need to include imported types because they may
contribute to cycles that invalidate a type.

This CL effectively reverts CL 202483 (issue #35049, which
doesn't apply anymore because we don't change the state of
imported objects).

Preparation for fixing issue #48962.

For #35049.
For #48962.

Change-Id: I06f15575ad197375c74ffd09c222250610186b15
Reviewed-on: https://go-review.googlesource.com/c/go/+/378675


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>