[!569] Let cmake disable search for modules with pkg-config.

Merge branch 'let-cmake-disable-pkgconfig-search' into 'master'

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.

See merge request [!569]

  [!569]: gitlab.dune-project.org/core/dune-common/merge_requests/569