go/types, types2: move validType code into its own file

The validType check is independent of the work of declaring objects.
Move it into a separate file for better separation of concerns and
code organization.

No other changes - this is purely a code move.

Preparation for fixing issue #48962.

Change-Id: Ib08db2d009c4890882d0978b278e965ca3078851
Reviewed-on: https://go-review.googlesource.com/c/go/+/378674


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>