haddock forces rebuild of empty packages

[snoyberg]

Steps:

1. Run stack haddock in the http-client repo (as an example)
2. When it succeeds: run it again

Expected: nothing happens

Actual: it rebuilds transformers-compat (presumably because there are no modules in that package), and then rebuilds everything that depends on it

[borsboom]

@snoyberg Fixed on master.

[snoyberg]

Confirmed, nice