Injecting/prioritising search results

[adamgundry]

Sometimes client code would like to insert additional results, typically in front of the full-text-search results. For example haskell/hackage-server@cf0f17e added a feature to hackage-server to prioritize exact package name matches. I've also got an application that does this by looking up query terms in a separate map. Of course the remaining results then need to be filtered to leave out those that have been prioritized.

It isn't clear if this needs to be in full-text-search itself or if it can just be implemented by the client. The implementation in hackage-server adds a new function lookupDocKeyReal, so we might need an equivalent of that at least.