cmd/compile: revert package typecheck part of CL 422914

cmd/compile: revert package typecheck part of CL 422914

This code path is unreachable anyway, and it adds new uses of
Type.Pkg, which shouldn't be used anymore.

Mark Type.Pkg as deprecated while here.

Updates #57410.

Change-Id: I1eec1c8ed99207d58d0ba0c44822bbad29dc64f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/458575
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>