When VTK is told to not use setup.py for installing Python (which is the case
with applications like ParaView), CMake rules were missing to ensure that the
python module libraries are installed. This patch adds those rules which are
used only when VTK_INSTALL_PYTHON_USING_CMAKE is ON.

Change-Id: I88afaae2a08915e02476ac1c1027fba4fc4fe3d4