[tox] Have less test environments This is to reduce the load on CI server because of parallel clones of the repository in Docker containers. Only keep "server", and "web" environments separated. All other prior environments are in "misc". To avoid duplicate entries in requirements files, move them all in a "requirements" directory (this appears to be a "common" practice, see e.g. Celery). Adjustments in tests: * Rename cubicweb/hooks/test/unittest_notification.py so that it does not conflict with cubicweb/sobjects/test/unittest_notification.py during test discovery as they would have the same module name but different __file__ attribute. * Add "comment" cube to the list of expected cubes in unittest_cwconfig.py as this cube is pulled by requirements/test-mist.txt. Closes #15440662.

[repo] Don't crash on start when fs schema is missing some db schema entities This occurs usually while developping and we don't want systematically to rebuild the database to start the instance or run e.g. i18ninstance.

Let configuration option be overridden by environment variables Related to #13889793.

[tox] Add a dummy test-results.xml file in check-manifest and flake8 environments This is to work around Jenkins Junit plugin that does not apparently account for the absence of this file (though it accepts it to be empty).

[tox] Basic flake8 config

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.

[skeleton] Fix modname in __pkginfo__

[test] Use plain unittest in unittest_cwctl.py