FindPythonLibs: Tolerate call with PYTHON_LIBRARY already a list

`PYTHON_LIBRARY` may contain a list because of `SelectLibraryConfigurations`.
If it is the case, the list can be:

    optimized;<FILEPATH_TO_RELEASE_LIBRARY>;debug;<FILEPATH_TO_DEBUG_LIBRARY>

Instead of directly using the value of `PYTHON_LIBRARY` in the CMake
function `get_filename_component()`, we loop over the content of the
relevant parts of `PYTHON_LIBRARY` and `PYTHON_DEBUG_LIBRARY` whether
they are lists or not.

Suggested-by: Brad King <brad.king@kitware.com>