*
     * This method must be invoked before {@link #addDependency(Node, Dependency, Predicate, Path)}
     * if output directories are desired on the class-path or module-path.
     * This method can be invoked at most once.
     *
     * @param main the main output directory, or {@code null} if none
     * @param test the test output directory, or {@code null} if none