[CMake][Exp PyROOT] Change default logic in the search for Python (CMake > 3.14)
[CMake][Exp PyROOT] Change default logic in the search for Python (CMake
> 3.14)

- Check if the PYTHON_EXECUTABLE deprecated variable was passed by the
user; if so, check whether it points to Python 2 or 3 and set the
appropriate Python{X}_EXECUTABLE variable
- Look for Python3 and set the deprecated variables to the ones set
automatically by find_package(Python3 ...)
- Look for Python2 and set the deprecated variables to the ones set
automatically by find_package(Python2 ...) ONLY IF PYTHON3 WASN'T FOUND

At the end of the search for Python 2 and 3, a series of lists
containing the pairs of Python CMake variables useful for MultiPython is
defined.

root-mirror/root,karies/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,karies/root