Rework the index creation process.

Pass 1 now iterates over the default hierarchy (either doc or the entire
libdir) and the help path (if enabled) to determine names and locations
of all help patches in the scope of the help browser (as set in the GUI
preferences). This initial pass is needed, in particular, to resolve
related object references in the meta data in pass 2.

Pass 2 then iterates over all index entries constructed in pass 1,
adding all the available meta data, including cross references to
related objects. When pass 2 finishes, index construction is complete
and the index cache is written to disk.

Note the new revision of the help browser narrows indexing to just the
-help.pd patches. Previous versions would index all .pd files. This may be
subject to review, but the new indexing scheme is faster and produces less
noise in the earch results (i.e., you won't see any helper abstractions or
other patches which just happen to be bundled with the help patches).