Make Links to Externally Defined Objects in Haddock Documentation Link to Hackage's Docs.

[TravisWhitaker]

Right now Haddock assumes that the documentation for our dependencies will be locally available, so all of the links to things that we don't define are broken. Ideally these would instead point to the appropriate Hackage Haddock page.

There's probably some Haddock option incantation that will accomplish this.

[gambogi]

This looks like it's possible from the raw haddock command. See documentation. Specifically, the --read-interface command looks like it might be what we want. Either way, this is blocking on an issue with the stack project. Arbitrary arguments are not currently passed through to subcommands: stack issue 864, though it presumably will be in the future.