Generic types can the source type of a type alias, so modify g.typ0() to
be able to deal with base generic types.
Added test aliasimp.go that tests aliasing of local generic types and
imported generic types.
Change-Id: I1c398193819d47a36b014cc1f9bb55107e9a565b
Reviewed-on: https://go-review.googlesource.com/c/go/+/322194
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
