move to full-text-search package.

[gbaz]

@adamgundry writes (in #615):

It might be worth switching Hackage to use the full-text-search package. This was derived from the Hackage search implementation (so it provides a very similar interface and switching should be fairly trivial), but it has a few extra features, notably autocomplete search.

[gbaz]

@adamgundry you mentioned at some point an interest in doing this yourself. Gentle ping to see if you're still potentially interested.