Hover docs including links for dependencies

As it stands, it seems that Haddock docs show up for boot libraries, and the project being developed, but not for third party libraries from Hackage.

I would guess (without really knowing anything about the current implementation) that if the dependencies have been installed with documentation (e.g. `documentation: True` in the global cabal config file), then the logic wouldn't be that different to the way it currently works for boot libraries - i.e. HLS just needs some way to know the path to the `.haddock` files.

I'd be keen to take a look into this if no one currently is. But I would need some pointers.