Auto detect main methods inherited from Java sources (including JEP 512 ones)

[Gedochao]

Follow-up to #4418 (comment)

This enables auto-detection of main methods inherited from Java classes/interfaces. Including ones added by JEP 512.

// C.java
public abstract class C {
    void main() {
        System.out.println("Hello from an instance main");
    }
}

// A.java
public class A extends C

Checklist

• tested the solution locally and it works
• ran the code formatter (scala-cli fmt .)
• ran scalafix (./mill -i __.fix)

How much have your relied on LLM-based tools in this contribution?

extensively

How was the solution tested?

added automated tests

[warcholjakub]

Two main things I found:

• First, not caused by this PR:
  We don't check whether the main method is protected (and it's not allowed for non JEP512 Java versions). And in result:

public class Test {
  protected static void main(String[] args) {
    System.out.println("hello");
  }
}

yields:

./mill -i scala run /tmp/test-repro/Test.java --jvm 17
[...]
Error: Main method not found in class Test, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application
924/925, 1 FAILED] /Users/jwarchol/Documents/GitHub/scala-cli/millw scala run /tmp/test-repro/Test.java --jvm 17 3s

I'll try to prepare a fix.
EDIT1: #4430
EDIT2: We'll probably have to make sure it works properly in inheritance too. So, imho, it'd probably be best to merge it before this PR, and address it here.

• Second, related to the changes:

package far;

public abstract class FarBase {
  static void main(String[] args) {
    System.out.println("FarBase.main");
  }
}

package near;

public abstract class NearBase extends far.FarBase {
  private static void main(String[] args) {
    System.out.println("NearBase.main");
  }
}

package near;

public class Child extends NearBase {}

results in Child being incorrectly classified as a main class.

[Gedochao]

Rebased on top of #4430 plus added handling for the missing case.
Should be ready to re-review.

[warcholjakub]

One more thing that I've overlooked previously is that when we take the example from my previous review and replace private static void main(String[] args) with public static int main(String[] args), then the same thing happens. But other than that - looks good.
Repro:

package far;

public abstract class FarBase {
  static void main(String[] args) {
    System.out.println("FarBase.main");
  }
}

package near;

public abstract class NearBase extends far.FarBase {
  public static int main(String[] args) {
    return 0;
  }
}

package near;

public class Child extends NearBase {}

[warcholjakub]

Is it ready for re-review?

[Gedochao]

Is it ready for re-review?

actually, yep!

[warcholjakub]

LGTM!