[dev.typeparams] cmd/compile/internal/types2: fix types2 panic

When reporting a "cannot import package as init" error, we can't rely
on s.LocalPkgName being non-nil, as the original package's name may
already be nil.

Change-Id: Idec006780f12ee4398501d05a5b2ed13157f88ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/320490


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