Move external packages out of `qubes` python module

[marmarek]

@woju (#1541 (comment))

Combining one python package from different distro packages is generally a bad idea. I wouldn't discount it altogether, however the need is very rare. Note that there are similar solutions like drop-in directories for plugins (vide /srv/salt/_modules), those however aren't proper python packages. In case of qubes I strongly advise against.

I also don't think we need a "naming policy" precisely because the names aren't changing anything (this wouldn't be the case if we put those modules in single package). The gain is purely cosmetic. That said, we could name all our packages as qubes*, i.e. qubessplitgpg, qubesimgconverter, and qubesmanager for that matter.

• qubes.mgmt - https://github.com/QubesOS/qubes-mgmt-salt-base-overrides/tree/master/src/qubes/mgmt
• qubes.xdg - https://github.com/woju/qubes-core-agent-linux/blob/master/misc/xdg.py