avoid nesting of sources for generated code #310

A potential fix for https://github.com/mpeltonen/sbt-idea/issues/310 by ignoring any sourceManaged that are sub directories of managedSources.
It's hard to say, what is right. Maybe the Play Framework shouldn't generate directly into `src_managed/main` (and it's really easy for them to fix it).

In any case, it's hard to address this problem without breaking someone's IDEA projects...

I would say: just add all the directories in `managedSourceDirectories` as separate source folder without any other magic (even without checking for source generators).