When an application writes

  find_package(VTK ... vtkMissingModule)

vtk_module_config (invoked from VTKConfig.cmake) should reject the
missing module if it is not enabled.  The current vtk_module_load check
incorrectly accepts modules that have been previously enabled in a build
tree but are no longer available.  Test against VTK_MODULES_ENABLED.

Change-Id: If0769b6192bf9c5878b7ef1e581d04df18b6bbef