[dev.regabi] go/types: must not import a package called "init"

This is a port of CL 287494 to go/types. The additional checks in
test/fixedbugs are included, though they won't be executed by go/types.
Support for errorcheckdir checks will be added to go/types in a later
CL.

Change-Id: I37e202ea5daf7d7b8fc6ae93a4c4dbd11762480f
Reviewed-on: https://go-review.googlesource.com/c/go/+/290570


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