Fix python meta data generation

Summary

Several smaller changes to improve python meta data handling, see dune-grid#141 (closed).

Main changes

• Move set(<module>_FOUND 1) to the end of dune_project()
• Add set(HAVE_<MODULE> 1) at the end of find_dune_project() to provide the "dune-way" of indicating found packages
• Collect all HAVE_<MODULE> flags for all dependencies in the python _cmake_flags variable