[Sema] Clean up extension binding a little
- Turn `BindExtensionsForIDEInspectionRequest` into the main extension
binding request.
- Change `ExtendedNominalRequest` such that it's no longer what
extension binding calls into to do the name lookup, instead it calls
directly into `computeExtendedNominal`. `getExtendedNominal` can
then be the entrypoint for `ExtendedNominalRequest` and assumes that
extension binding has already run. This avoids needing to fake the
dependency relationship in the DeclChecker.
