'Given a sort order specified by the pkgname/component/second_key,
return a list of CompilerSpecs, VersionLists, or Specs for
that sorting list.'

@classmethod
def _specs_for_pkg(cls, pkgname, component, vpkg=None):

    key = (pkgname, component, vpkg)
    specs = cls._spec_cache.get(key)
    if (specs is None):
        pkglist = cls._order_for_package(pkgname, component, vpkg)
        spec_type = _spec_type(component)
        specs = [spec_type(s) for s in pkglist]
        cls._spec_cache[key] = specs
    return specs
