move to full-text-search package.

@adamgundry writes (in https://github.com/haskell/hackage-server/issues/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.

Start by reading the current Hackage search implementation referenced in issue 615 and comparing its interface with the full-text-search package. Confirm how the package's autocomplete feature fits the existing search behavior. Done means Hackage uses the package while preserving the similar search interface and exposing the relevant extra functionality.

Written by the indexing model from the issue text.