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.
I was going back and forth on adding `sourceManaged` or adding each `managedSourceDirectories`. 
Given the sbt documentation only says that generated sources "should" (and not must) go in a subdirectory of `sourceManaged` I went with `managedSourceDirectories`.

Generated java code which would not work at all is a much stronger constraint than "should vs must", so I agree `src_managed/main` should be added as source root.