Cleanup and updates for URIs and URLs.

* [URI_EXT,SC] Renamed url_directory/3 -> url_flat_directory/3.
* [URI_EXT,SC] Renamed url_to_directory_name/2 -> url_nester_directory/3.
* [URI_EXT,SC] Renamed url_to_file_name/3 -> url_nested_file/3.
* [URI_EXT,SC] Renamed url_to_graph_name/2 -> url_rdf_graph/2.
* [URI_EXT,NEW] Added uri_component/3 wrapper for library URI predicates.
* [URI_EXT,NEW] Added relative_url_path/3, based on relative_file_path/3.