python: expose PYTHON_PATH

As a preparation to make the Python infrastructure support both Python
and Python 3, as well as the bump of Python 2 and 3, we need the
Python package to expose the Python module path in a variable called
PYTHON_PATH. It will be used by the following commits.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>