Installation of pkg-config / cmake files depend on libdune<module>

If a Dune module provides a library (like common, geometry, grid), pkg-config and CMake files are installed below the library path (e.g. lib64). If a module provides no library (like localfunctions, typetree), the same files go below a different lirary path (e.g. lib).

Caused by this part of our code: https://gitlab.dune-project.org/core/dune-common/-/blob/master/cmake/modules/DuneProject.cmake#L246-L255

Is this really our intention? Why not always using one of the two variables CMAKE_INSTALL_LIBDIR or DUNE_INSTALL_NONOBJECTLIBDIR?