def _make_tree_defs(mod_files_list):
    """get a list of 2-uple (module, list_of_files_which_import_this_module),
    it will return a dictionary to represent this as a tree
    """
    tree_defs = {}
    for mod, files in mod_files_list:
        node = tree_defs, ()
        for prefix in mod.split('.'):
            node = node[0].setdefault(prefix, [{}, []])
        node[1] += files
    return tree_defs
