pkg-python: use the newly defined PYTHON_PATH variable

Now that the Python package exposes its PYTHON_PATH variable, we can
use it in the package infrastructure. This prepares both the upcoming
bump of Python 2.x, and the introduction of Python 3 support in the
Python package infrastructure.

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