move haddock library to a separate haddock-api package

This patch moves the haddock library to `haddock-api` and adds a new `haddock` package with just the executable in the `haddock-executable` subdir.

I added a `build-executable` flag (manual) so it's still possible to get the executable with the `haddock-api` package, for testing.

I also exported a few more functions from `Documentation.Haddock`, required to build `haddock-ghcjs`


2.14.4 is not a major version bump and there may be other users than those with a package on hackage. I think it's just doing the right thing not unnecessarily breaking things in a minor version bump. Releasing this as 2.15 without lib would be equally fine in my opinion.

Good point about `exposed: False`, added that.