Support test sources/dependencies

[charleso]

Currently the plugin only looks at unmanaged sources. Adding the paths is easy enough, but changing the current mkDerivation will need some thought. It could possibly look something like:

sbt.mkDerivation {
   compile = {
      sources = [ ... ];
      buildDepends = [ ... ];
   };
   test = {
      sources = [ ... ];
      buildDepends = [ ... ];
   }
}

And return a set rather than a derivation (so we should probably change the name), and allow the caller to run compile or test appropriately.