Update cmake rules following vtk-modular changes for Python.

Updates to use the newly added infrastructure in VTK to deal with
VTK-modules that are simply Python modules/packages.

As with VTK, we needed to move the Web/Python/*.py files under a
directory structure that reflects the target package directory
structure.

Change-Id: Ia63ac73c8e0270a13a478e70ff1677e0ac45d500
Merge topic 'fix_python_packages_and_modules_in_vtk_modules' into next

7f8505cf Update cmake rules following vtk-modular changes for Python.