go/types, types2: add a test for invalid import of "init"
This error reporting code path did not have test coverage, and panics in
1.17 (filed as #48082).
Add a test that would have reproduced the panic, for both go/types and
cmd/compile/internal/types2.
Change-Id: Icd5f54f8407e4ab57d432f44a129ecf6b2755feb
Reviewed-on: https://go-review.googlesource.com/c/go/+/346309
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>