Fix #2198: Don't widen module singletons

[odersky]

Since module classes are a compiler-generated construct that's not directly
visible to programmers, it seems better not to automatically widen a module
singleton to its underlying class.

Fixes #2198.

[liufengyun]

Looks good to me, but what about case val which are not modules defined by enums?

[smarter]

LGTM also, you should be able to revert the code change in #1448 but keep the test and it should still work.

[odersky]

I'd leave #1448 in. It's still a useful subtyping relationship to have.

[odersky]

@liufengyun Good question about enum vals. Here's what, e.g. None translates to:

final case val None: Option[Nothing] = 
  {
    final class $anon() extends Option[Nothing]() { 
      def isDefined: Boolean = false
      def enumTag: Int = 1
      override def toString: String = "None"
    }
    new Option[Nothing]{...}(): Option[Nothing]
  }

I think there's nothing to do. None is an Option[Nothing], and should be widened to that. This is analogous to the change in enum classes where apply also returns instances of the enum class itself instead of soemthing more specific.