[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.

Handle cubes as packages in cwconfig Rely on importlib.import_module in several places in cwconfig.py, so this would not work with python 2.6. Method available_cubes will not list cubes installed as package for now. I'm not sure it's worth implementing this method (used in `cubicweb-ctl list cubes` command) for new cubes layout as the same result can basically be obtained with `pip freeze | grep cubicweb_`. In unittest_cwconfig.py, duplicate CubicWebConfigurationTC test case to test both the "cubes as packages" layout and the "legacy cubes" layout. The former having a custom sys.path set (pointing to datapath('libpython') where all cubes' packages live) and the latter having the previous config attribute setup. All test data cubes are moved to packages in libpython directory and symlinks are introduced in the cubes directory. Related to #13001466.

[cwconfig] Reorder imports by alphabetic order

Add an import redirect hook from "cubes.<name>" to "cubicweb_<name>" The hook consists of a finder and a loader implemented following PEP-302; it is responsible for loading cubes distributed as packages (i.e. installed as ``cubicweb_<name>`` in site-packages) but imported (in client code) as ``from cubes.<name> import ...``. So this is a transitional mechanism allowing cubes following the new layout to be used by old-style cubes/applications. The importer is installed upon calling CubicWebConfiguration's cls_adjust_sys_path method (also called in cubicweb.devtools.__init__.py, which is a prerequisite for importing any "legacy" cube. The loading of old-style cubes is still handled by the CubicWeb configuration, based on adjustment of sys.path etc. Related to #13001466.

[devtools] Update skeleton's setup.py to install cube as a package Most of the prior logic of skeleton's setup.py gets dropped as installing a cube as a "classic package" (i.e. in site-packages) is just the default behavior of distutils. Also add a test checking installation of new cube. Related to #13001466.

[devtools] Adjust skeleton setup.py to have it read information from __pkginfo__.py Since the later is now installed in cubicweb_<CUBENAME> directory. Add a test ensuring that `python setup.py sdist` works on newly created cube (which should at least verify that setup.py is valid). Related to #13001466.

[devtools] Update skeleton and newcube command to cube as package layout Related to #13001466.

[pkg] Add missing entries to MANIFEST.in Detected by running `check-manifest <https://github.com/mgedmin/check-manifest>`_. Add a tox environment accordingly. Closes #15229018.

[test] Add a test for toolsutils.read_config

[test] Use plain unittest in unittest_toolsutils