[skeleton] do not import the __pkginfo__.py module to make cubes installable via setuptools
When using a simple "import" statement, running 'python setup.py install' for
a cube fails in a fresh virtualenv due to the fact the install of the
dependencies (here cubicweb iteself) is done by the python process doing the
cube's installation, so the __pkginfo__ module from the cube is already
loaded (thus used) when installing cubicweb...
So we apply the third technique from:
https://packaging.python.org/en/latest/single_source_version.html