'mixin' methods not generated for case objects

[raboof]

When a method is defined in a trait (i.e. TraitWithMethod), and that trait is mixed into an object (i.e. MethodFromTrait:

trait TraitWithMethod {
  def theMethod(): Int = 42
}

case object MethodFromTrait extends TraitWithMethod {
  def foo(): Unit = ???
}

Then this should lead to 3 class files: the trait is converted to an interface declaring the method:

public interface TraitWithMethod {
  public static int theMethod$(TraitWithMethod);
  public int theMethod();
  public static void $init$(TraitWithMethod);
}

The MethodFromTrait$ class that implements the interface and contains the method implementation:

public final class MethodFromTrait$ implements TraitWithMethod {
  public static MethodFromTrait$ MODULE$;
  public static {};
  public int theMethod();
  public void foo();
}

And a MethodFromTrait class with static forwarders:

public final class MethodFromTrait$ implements TraitWithMethod {
  public static MethodFromTrait$ MODULE$;
  public static {};
  public int theMethod();
  public void foo();
}

Currently, genjavadoc does not generate the method on MethodFromTrait$, because genjavadoc runs after the fields compiler stage, and the method is generated later in the mixin stage.

We could fix this by either running genjavadoc after a later compiler stage (but this potentially means undoing a lot of the work in those phases, or simulating the mixin stage logic in genjavadoc.