Finally came up with noun names for the indexers!

Now we can think of them as some kind of lazy trees or lazy files, rather than the more abstract and thus harder to think about "things that index".

Also, morsels -> needles. You search for a needle in a haystack. You don't search for morsels.
Factor the filter-finding out of Plugin.
Port buglink plugin to new API.
Remove the "qualname" item from the tuple returned from refs().

Nobody ever used it except to put it on a CSS class which was never styled or otherwise referred to.
Group found URLs by line as the API requires.

Also simplify URL regex. We were going through an awful lot of trouble to include any surrounding [] or () in a link's href.