Rename and rework repo.py and client.py

[cminyard]

repo.py and especially client.py are really terrible names for those scripts. However, repo.py is a useful tool by itself and for use by other scripts.

I propose to move repo.py and client.py down a level into the tuf directory and create tuf-repo and tuf-client scripts that call into them. I can generate a patch, if you like.

[joshuagl]

This issue should probably be closed in favour of #881, it's effectively a duplicate (though this issue was chronologically duplicated by #881, the latter has a lot more discussion).

[lukpueh]

Closing in favor of #881