Use pkgutil.ImpLoader instead of a custom loader for cubes import redirection It just works fine (same implementation of load_module method as _CubesLoader introduced in d404fd8499dd) and is complete w.r.t. PEP 302 (all methods implemented). Related to #13001466.

[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.

[skeleton] Use distname directly in setup.py distname is part of required metadata and modname cannot actually be used in place of it.