go/types: type-check embedded methods in correct scope (regression)

Change https://go-review.googlesource.com/79575 fixed the computation
of recursive method sets by separating the method set computation from
type computation. However, it didn't track an embedded method's scope
and as a result, some methods' signatures were typed in the wrong
context.

This change tracks embedded methods together with their scope and
uses that scope for the correct context setup when typing those
method signatures.

Fixes #23914.

Change-Id: If3677dceddb43e9db2f9fb3c7a4a87d2531fbc2a
Reviewed-on: https://go-review.googlesource.com/96376


Reviewed-by: Alan Donovan <adonovan@google.com>