[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.
usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl