Remove duplicates in ALL_DEPENDENCIES

The legacy way to generate a list of found dune module dependencies is by dune_create_dependency_tree(). In the process of cleaning this up and moving the dependency creation into the config.cmake files, the order in that list was reversed and no duplicates are created. In order to be consistent, this MR performs the reverse+unduplicate step after constructing the tree. In this way, the modules in the legacy path are correctly listed in the right order with modules more up in the tree in front of those more down in the tree.