Mixed routing file paths in the router

In the find_routing_files/find/get_routes method, the paths are mixed (some are relative to the phpBB root and some to the current dir). So when we are in a sub directory and we need to use the router (the acp login form per example) an exception is thrown.

We should convert all the paths to be relative to the phpBB root path (ie: not prefixed by $phpbb_root_path).