Correctly deal with Java/Scala impedence mismatch between Object/Any when using -Ypickle-write-java

[retronym]

The specific case that I believe is caused by this is:

package demo;

public class C {
  public static void ol(scala.Equals o) {} // Equals extends AnyVal
  public static void ol(Object o) {}
}

package demo
class Client {
  C.ol(Option(""))
}

$ for V in 2.12.15 2.13.6; do echo $V; scalac --scala-version $V -d /tmp -Ypickle-java -Ypickle-write /tmp/c.jar /tmp/C.java && scalac --scala-version $V -cp /tmp/c.jar /tmp/Client.scala; done
2.12.15
2.13.6
/tmp/Client.scala:3: error: ambiguous reference to overloaded definition,
both method ol in class C of type (o: Object): Unit
and  method ol in class C of type (o: Equals): Unit
match argument types (Option[String])
  C.ol(Option(""))
    ^
1 error

The relevant changes in 2.13 were:

scala/scala#7966
scala/scala#8049

-Ypickle-write is part of the infrastructure to support build pipelining. It writes a .sig file for .java source file that has its API typechecked by the Scala compiler. This can be uses on the classpath of downstream Scala compilation, instead of .class files.