Don't register include directories in dune registry of packages

dune_project is eager to register include directories of upstream modules into the registry of packages. The problem with this is that include directories must be guarded with proper build and install interfaces to work well. If an unguarded include directory is added to an installable target, cmake will rightfully complain that this will not work on the installed version. A first iteration of this MR pointed at out that the right solution here is to just register the target, and delegate the include directories to whoever creates the target (we will add a helper for this later in !1355 (merged)). However, in the discussions we realized that this is also not necessary due dune_project invoking include_directories on upstream packages anyways, making this line completely superfluous in single-builds and instead obstructing super-builds.

cmake/modules/DuneModuleDependencies.cmake

@@ -201,7 +201,7 @@ macro(dune_process_dependency_macros)
       endif()
 
       # register dune module
-      dune_register_package_flags(INCLUDE_DIRS "${${_mod}_INCLUDE_DIRS}")
+      dune_register_package_flags(LIBRARIES "${${_mod}_LIBRARIES}")
     endif()
   endforeach()
 endmacro(dune_process_dependency_macros)