R1:f4f73e0a7e46

Separate fulltext engine extensions into "enrich" and "index" phases

Summary:
Ref T12819. Some of the extensions "enrich" the document (adding more fields or relationships), while others "index" it (insert it into some kind of index for later searching).

Currently, these are all muddled under a single "index" phase. However, the Ferret extension cares about fields and relationships which other extensions may add.

Split this into two phases: "enrich" adds fields and relationships so other extensions can…