Changes for https://github.com/ceylon/ceylon-ide-eclipse/issues/567 :

- Move model-less-packages-related methods into AbstractModelLoader
- Use AbstractModelLoader in the Compiler classes (CeylonEnter, ...)
- Rename PhasedUnitsManager to CompilerDelegate
- Rework a bit the CompilerDelegate (methods changed)
It should be an exactly-equivalent refactoring.