CMake module path not properly setup at config time

One of the objectives of !1249 (merged) is to be able to include downstream modules directly without the need to find all upstream dependencies manually, e.g. call find_package(dune-grid) without a previous call to find_package(dune-common). Nonetheless, this does not work yet due to the cmake paths not being properly set up at config time. See an example here where dune-copasi config is found but it fails to find dune-common cmake modules needed to find all upstream modules. This was initially well set up in my original MR but I had to remove it due to some incompatibilities with cmake modules in other unrelated packages (see whole argument here !1249 (comment 134581)).