db/simple/Song: rename "uri" to "filename"

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.

src/db/plugins/simple/Directory.cxx

@@ -191,7 +191,7 @@ Directory::FindSong(const char *name_utf8) const noexcept
 	for (auto &song : songs) {
 		assert(&song.parent == this);
 
-		if (song.uri == name_utf8)
+		if (song.filename == name_utf8)
 			return &song;
 	}