Assignment plugins is not completely plugable

When developing a custom assignment plugin (random), its name cannot be found in directories searched. More explicitly, function assignment_types in assignment/lib.php calls function get_string('type'.$name, 'assignment'). All official plugins get string name typeonline, typeoffline, etc from official language file asignment.php in lang/en_utf8 (or other appropriate language). However, custom plugin should get its strings from mod/assignment/type/random/lang/en_utf8. Although language file assignment_random.php in this directory is browsed for other strings, string typerandom is not read from it. This results in [[typerandom]] messages in list of assignments and at the assignment update pages.