Move DirectoryIndexer-specific code out of base IndexWriter

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
DirectoryIndexer: rename misnamed method

The second "index()" method really discovers the annotations and does not
write any annotation index.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Add a helper for Eclipse

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>