[config] Make available_cubes aware of cubes installed as packages
For this add a "cubicweb.cubes" entry points group which will be scanned
through to find out installed cubes. Entries in this group are expected to
expose the module name of a cube (i.e. `cubicweb_foo`).
Note that CubicWebConfiguration's available_cubes method will return the
module name of cubes as packages (cubicweb_foo), so we had to add a special
"key" sorting function to keep cubes sorted as before, despite possible
different distribution schemes. This makes it possible to handle loading of
CTL plugins in an almost similar manner as before (just tweaking the package
name from cube name in load_cwctl_plugins method).
I had to tweak (again?) the test_cubes_path method in unittest_cwconfig.py but
did not find out why.
Apart from unforeseen bugs and pending documentation, this finishes the work
on porting cubes to standard Python packages. Closes #13001466.