go/types, types2: eliminate methodList in favor of just using Named.mu

go/types, types2: eliminate methodList in favor of just using Named.mu

In order to clean up context after fully expanding a type (in subsequent
CLs), we must use a common mutex. Eliminate the lazy methodList type,
which keeps a sync.Once per method, in favor of Named.mu.

Updates #52728

Change-Id: I2d13319276df1330ee53046ef1823b0167a258d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/404883
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>