Create python::pil; relax restrictions on packages.pp

- The rule that packages must be declared with no parameters is too strict for
  packages installable via pip, since those must at least specify provider =>
  'pip'. So relax the restriction for now.
- At the same time, the setup for PIL really is a bit too involved for
  packages.pp, so move it to a python::pil class in its own module.

Change-Id: I575809a87b62346b11d254a8a1d1e72daf9a9e24