Use version from package in setup script

Now that we have a package, we keep track of the version number there.
We import it to use it in the setup script.
Update setup script for new package layout

Now that we use a package (c915ddd), the setup script
has to ba adapted to correctly dispatch the modules.

Also, we do not have a script anymore: we rely on entry_points
to point to the main method of commonsdownloader module.