Fixed warning H302 when used with six.moves

Checking whether something is a module now also consults ``sys.meta_path``, as
per the import specification. This fixes H302 with six >= 1.7.0, where a custom
finder/loader is used.

Change-Id: I4e401e0fcce077f81a5471692aa6ecd16f26f6ed
Close-bug: 1329002