Let cmake disable search for modules with pkg-config.
For whatever reason we still allow to search for a module with pkg-config if CMake was not successful. This leads to the weired situation that if we pass -DCMAKE_DISABLE_FIND_PACKAGE_dune-uggrid=ON to cmake and dune-uggrid is installed in a default sytem path we will end up with a CMakeCache.txt actually using dune-uggrid.
This patch is the minimal change to stop searching for a DUNE module with pkg-config if CMake was instructed not to use it.