[dev.typeparams] cmd/compile: get type aliases working with generic types

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>