Register module libraries instead of include directories

The problem is that include directories must be guarded with proper build and install interfaces to work. If an unguarded include directory is added to an installable target, cmake will rightfully complain that this will not work on the installed version. So the right solution here is to register the target, and delegate the include directories to whoever creates the target (we will add a helper for this later)

Note that once !1355 is merged, targets calling dune_target_enable_all_packages will also nicely propagate these dune packages across modules even if cmake projects do not use the dune build system.