cmd/compile/internal/noder: remove unified IR assumptions on LocalPkg.Path == ""

cmd/compile/internal/noder: remove unified IR assumptions on LocalPkg.Path == ""

Replace `pkg.Path == ""` check with `pkg == types.LocalPkg`. This is a
prep refactoring for CL 393715, which will properly initialize
types.LocalPkg.

Updates #51734.

Change-Id: I7a5428ef1f422de396762b6bc6d323992834b27c
Reviewed-on: https://go-review.googlesource.com/c/go/+/406056
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>