[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.
usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/