[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.