go/types, types2: move go/types-only Scope methods into scopes2.go

go/types, types2: move go/types-only Scope methods into scopes2.go

Remove them them from types2.

Updates #69673.

Change-Id: I7843f6da1edf3a19f85c61706104d173e04088d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/616261
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>