go/types, types2: use same method lookup code in both type checkers

types2 has some code to handle case-folding when doing method lookups
which was missing in go/types. This change is a first step to match
the implementations. Specifically:

In types2:
- remove the lookupMethodFold names in favor of just lookupMethod,
  but with the foldCase flag (e.g., instead if lookupMethodFold,
  we just use lookupMethod)
- rename checkFold to foldCase everywhere
- add foldCase parameter where it was missing
- moved foldCase paremeter to the end in lookupFieldOrMethod
- no functionality changes

In go/types:
- match function signatures with types2 use
- always provide false as argument for foldCase for now
- no functionality changes

Preparation for fixing some of the outstanding error reporting issues.

Change-Id: If129a5feb89ddf96a3596e8d73b23afa591875a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/382461


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