go/types: unexport Checker.LookupFieldOrMethod
    
    Implementation changes in go/types for #6977 required that internal
    LookupFieldOrMethod calls had access to the current *Checker. In
    order to make quick progress, I added a *Checker receiver to the
    function LookupFieldOrMethod (thus making it a method), and added
    a new function LookupFieldOrMethod. The plan was always to rename
    that function (Checker.LookupFieldOrMethod) such that it wouldn't
    be exported; with the obvious name being Checker.lookupFieldOrMethod.
    But that name was already in use which is why I postponed the rename.
    Eventually I forgot to clean it up. This CL fixes that with the
    following renames:
    
    Checker.lookupFieldOrMethod => Checker.rawLookupFieldOrMethod
    Checker.LookupFieldOrMethod => Checker.lookupFieldOrMethod
    
    Updates #6977.
    Fixes #36916.
    
    Change-Id: Icfafd0de9a19841ba5bd87142730fe7323204491
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217134
    Run-TryBot: Ian Lance Taylor <iant@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
    Reviewed-by: Ian Lance Taylor <iant@golang.org>