[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

[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.

Drop the only reference to an '__insert' form key

Drop bits from the past (no more dbapi isolated installation)

[devtools] CWTC.request is gone, don't attempt to use it in CWTC.view So one will have to provide at least one of `rset` or `req` argument of the `view` method.

Merge 3.22 into 3.23 Some fixes there are necessary for client work depending on cubicweb 3.23.

[devtools] Handle i18ncube command for "cubes as packages" And add a test so that both layouts are tested. Move the test cube into a package. Related to #13001466.