Public constructors for singletons leak into class files

[anatoliykmetyuk]

object Foo

When compiled with Dotc:

MacBook-Pro-Anatolii:dotty anatolii$ javap Foo$.class
Compiled from "Playground.scala"
public final class Foo$ implements scala.Serializable {
  public static final Foo$ MODULE$;
  public static {};
  public Foo$();
}

When compiled with Scalac:

MacBook-Pro-Anatolii:issues anatolii$ javap Foo\$.class
Compiled from "Playground.scala"
public final class Foo$ {
  public static Foo$ MODULE$;
  public static {};
}

This breaks logic that depends on class file analysis. E.g. mill test discovery.