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 have a scenario where Play Framework generates the `routes.java` file in `src_managed/main/controllers` folder.

Note that `controllers` is the package of the class `routes`. Therefore, the root source directory must be `src_managed/main` instead of `src_managed/main/controllers`.

I would say that the correct behaviour is to list only `src_managed/main` as the source root folder and not list any subdirectories. See pull-request #318