go/types, types2: pull up package-level object sort to a separate phase

go/types, types2: pull up package-level object sort to a separate phase

This step allows future additional phases to reuse the sorted object
list. Preparation for upcoming CLs.

Change-Id: I22eaffd5bbe39c7cc101c6d860011dc3cb98ce37
Reviewed-on: https://go-review.googlesource.com/c/go/+/715480
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>