Steal Stackage's search

The [Stackage](https://www.stackage.org/) site has a search that combines `hoogle`, package search, and module search. So if I want to see the docs for `Control.Concurrent`, I type that in, and I'm there. If I want to see the docs for some function, I search for the function, and it comes up.

I've noticed that I'm starting to search with Stackage first, which is unfortunate, as stackage does not have all the libraries that Hackage has. Would it be possible to incorporate these features into Hackage's search mechanism?

Start by reviewing Hackage's existing search mechanism and comparing it with Stackage's combined search. Done would mean Hackage search can find packages, modules such as Control.Concurrent, and functions through Hoogle-style queries, covering libraries beyond Stackage.

Written by the indexing model from the issue text.