Fix setuptools installation

[chrisburr]

Currently pip install results in the scripts like dirac-proxy-init being installed as dirac-proxy-init.py. This is relatively simple to fix in the setup.py file however there is still a deeper issue when using extensions. In LHCbDIRAC the dirac-dms-add-file script is overridden and when pip installing both DIRAC and LHCbDIRAC, the installations will overwrite each other's files making it "random" as to which one is available.

The "correct" solution is probably to use entrypoints.

(See also #3523)

[chrisburr]

@andresailer @atsareg @chaen @remenska @fstagni @sfayer and anyone else who's possibly interested.

I've written down a proposal for how properly integrate DIRAC with setuptools at this link. Feedback would be particularly useful for the "Duplicated entrypoints" section and the handling of multiple VOs/extensions. If there are no strong objections, I'll open a PR at the beginning of next week with the initial implementation.

[fstagni]

I'll bring this up at tomorrow's BiLD meeting. @chrisburr you can briefly go through the points there, then we'll all need to do the homework anyway.