Fix the output directory

According to the documentation of the recent version of sbt, the output directory should be a subdirectory of the `sourceManaged in Compile`, not the `sourceManaged in Compile` itself.
For multiple projects, sbt-idea excludes the directory from the list of source directories without conforming to this.

References:
    http://www.scala-sbt.org/0.13.1/docs/Howto/generatefiles.html
    https://github.com/mpeltonen/sbt-idea/issues/209
    https://github.com/sbt/sbt-scalabuff/pull/6