Implement new pypi mirroring algorithm.

* jeepyb/cmd/run_mirror.py: Overhauled script, implementing new
algorithm to catch transitive dependencies. Additional output is
provided to indicate what's being cached for each project, and a
debug level has been added as well. Some previously unused codepaths
were removed for improved simplicity and clarity.

Change-Id: Id4ec4c5eb4420f6db972fcd5b6bbb1a5047e21d2
Reviewed-on: https://review.openstack.org/23397
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins