Move to PythonPath-based installed extension registration

Right now, downloaded python package are injected into the classloader, since Jython scans the classloader to find them.

It's not the case of GraalPy which takes a more comm Python approach and expect the path of each python package to be listed as value of the "python.PythonPath" property.

What this means for the egg/wheel packages support is that instead of injecting the packages in the classloader, then it should just add/remove the paths from the local extension repository to a list which is then set as "python.PythonPath" value.