Module macros cleanup.

Projects sometimes want to add modules directly i.e. have the vtk_module(..) and
vtk_module_library(..) calls directly added in the same CMakeLists.txt file
without using a module.cmake at all. This is needed especially for modules that
tend to depend on "all enabled" modules. This change ensures that
vtkModuleMacros work even when module.cmake is missing.

Change-Id: I11122a330cb31b3f8785435fddd44d44df9bd236
Merge topic 'extend_modules' into master

8a045b49 Module macros cleanup.