[Haskell-cafe] Hide internal modules but expose them for testing

Greetings, I want to have internal modules which I do not expose at all. I also want to be able to test them. This however also hides these modules from my tests. The best way I have found to work around this is to include my entire source directory in the test suite (source-dirs). This however means that I can't reuse builds that didn't include the tests. Is there some obvious and "best" solution that I am missing here? For reference I use Stack. Thanks in advance! ======= Georgi