pip 10 moved several interfaces to an internal package, causing our pyversion.py script to fail.
This patches fixes the issue by getting installed distributions directly from setuptools, as
suggested in https://github.com/pypa/pip/issues/5243.

Thanks to Martin Nolte for noticing.

So far present:
* The entry-level module DunePythonCommonMacros.cmake
* The check for the interpreter
* A function dune_python_find_package to check for installed packages