Unqualified reference to module constant from extern-ed class doesn't compile in Java

[robin-aws]

Repro:

module {:extern} M {
  const FOO := 5

  class R {
    var v: int
    constructor() {
      v := FOO;
    }
  }
}

The Dafny-to-Java compiler produces __default.FOO for the constant reference in the constructor, which doesn't resolve because the FOO constant is defined in M_Compile._ExternBase___default.java. The fix is to manually define an empty __default class:

package M_Compile;

public class __default extends _ExternBase___default {
}

This is a side-effect of the mechanism for mixed Dafny/Java classes as described in #469, but calling this out separately because it's especially difficult to recognize that modules need the same empty subclass.