Consider deferring module top level in type checker

We currently never defer module top levels in type checker. This can cause some Cannot determine type of x errors in import cycles. They are real in some sense, because such code would almost certainly fail at runtime. However, this might sometimes be useful in stubs (as practice shows, this is however not important and shows up only in highly entangled import cycles, such as e.g. builtins SCC).

No se nombran archivos, pruebas ni puntos de entrada. Empieza rastreando el manejo de nivel superior de los módulos en el comprobador de tipos y reproduce los errores reportados Cannot determine type of x en un ciclo de importación, incluido el caso de SCC de builtins; la tarea requiere que se haya decidido una política de aplazamiento y que haya pruebas que muestren el comportamiento previsto del stub.