This teaches the FindVTK module to use the Config mode of find_package()
to search for VTKConfig in the common case.  The old search method based
on find_path() and UseVTK is now used only to search for VTK 4.0.

This approach avoids the need to update the module for each new VTK
version because find_package(VTK) automatically searches "lib/vtk*".  It
also addresses issue #9105 since find_package searches lib64 paths too.