From what I see, haddocks aren't built when installing dependencies.

It would be nice to support these use cases:

1. I only want to install an app and don't care about haddocks, don't waste my time
2. I have plenty of time, build haddocks while installing packages
3. I want to get started ASAP, so for now, only build the dependencies, but later I may want to run stack haddock and have it build the haddocks for the dependencies (without rebuilding them, of course). In other words, be able to build haddocks on demand.