Put all wrap hierarchy files in VTK_MODULES_DIR

This allows for much simpler code when wrapping an external VTK module.
Also just check if the module is wrap hierarchy excluded - the
<module>_BINARY_DIR will not exist when building against VTK.

Change-Id: Id6e92685239dddab3c4a8911aea4017e349b8942
Check if the PythonD target exists

This allows for external Python wrapping to work, and also continues to
work as expected when wrapping VTK modules. The dependency source
directory was also being added twice (once here, and once with the
binary directory in CMake/Wrapping.cmake).

Change-Id: Ie33ae695b66655b2b7c424f08c55cb18f25d882e
Merge topic 'wrapping-external' into master

a55006c1 Check if the PythonD target exists
c432ae16 Put all wrap hierarchy files in VTK_MODULES_DIR