Discard files in external builder

Hi,

I'm writing an external builder for Ceylon. The ceylon compiler is built on top of javac, so it's able to compile both *.java and *.ceylon files in the same project. Since Java files can reference Ceylon declarations from the same project, I'd like to call the ceylon compiler first, then make the java compiler ignore Java files which were compiled by Ceylon.

To do this, I have a `ModuleLevelBuilder` with category `SOURCE_PROCESSOR`, so it's called before the Java builder. Now, I need to tell the Java builder that it should ignore some dirty Java files, because they were already compiled. How can I do this?

As an alternative, is it possible to avoid calling the Java builder on modules which have a certain facet? In my case, if a module has the Ceylon facet, it will automatically be compiled by the Ceylon builder.

Thanks.

Bastien