[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.

[skeleton] Fix modname in __pkginfo__

[test] Use plain unittest in unittest_cwctl.py

[devctl] Remove commented LiveServerCommand The code got commented in 2010 (changeset 5ab3f63b06ad)...

[pkg] Inline call to setup() in setup.py The point of having this "install" function, called in "main" mode is not clear. Better stick to standard practices.

[pkg] Drop old checks in setup.py

[tox] Exclude symlinks from check-manifest call These are not handled by check-manifest, see https://github.com/mgedmin/check-manifest/issues/69.