go/types, types2: use identical missingMethod in both type checkers

Further simplify and regularize Checker.missingMethod and use the
same code in both type checkers.

This enables case-folding lookup for go/types.

Adjusted test case that looks for alternative methods.

Change-Id: I5b8cc598c295c329ff93b1c65787cc6140f0900e
Reviewed-on: https://go-review.googlesource.com/c/go/+/382858


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>