Supress duplicate setup_requires in mirror.

Use an unordered set to ensure that when we add setup_requires
to the list of frozen requirements we don't have duplicates.

Change-Id: I1f11cfe52b6732b5d7dc24b687b6f9f7ba3ffb99
Reviewed-on: https://review.openstack.org/24949
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins