Right now, a playlist with absolute pathnames can only add songs that
are in the same the directory of the playlist or under it.

If uri is an absolute pathname and base_uri is set,
playlist_check_translate_song() will check that base_uri is a prefix
of uri, excluding every other song in the music directory outside
base_uri.

I think in this case base_uri should be completely ignored (and made
NULL) and uri should just be checked against music root directory.