Tweaks to allow compilation in IntelliJ

 - Move ScaladocModelTest to partest extras (the scaladoc module
   should not directly depend on partest.)
 - Remove the scala facet from the 'manual' module, we don't
   need to compile that code other than when creating a distribution.
 - Add a project library containing `build/deps/scaladoc` and add
   this to the classpath of the `scaladoc` module.