Filters cannot be added as symbolically linked directories

We use symbolic links when adding some plugins to Moodle sites which works fine for most plugin types (like mod, blocks, auth, etc...) but does not work for filters.

The functionality in lib/filterlib.php that looks for filter plugins uses the get_list_of_plugins() function in lib/moodlelib.php, which is not seeing symbolic links as being a valid directory (using php function "filetype"). The other plugin types work because they are generated via function get_plugin_list() which uses php function "is_dir" which does identify the symbolically linked directories as valid directories. The get_list_of_plugins function should be changed to allow symbolic links (or then modify the filter type logic to use get_plugin_list like the other types).