Incorrect overriding can mask implementation obligations in subclasses

[tdegueul]

CLASS_NO_LONGER_CONCRETELY_EXTENSIBLE may miss a subtle case where a package-private abstract method is inherited through a public abstract class in another package:

package p;
public abstract class A {
    void m() {} // Becomes abstract in v2
}

package q;
public abstract class B extends p.A {
    public void m() {}
}

Both API versions compile because q.B is abstract. However, this compiles against v1 and fails against v2:

package client;
public class C extends q.B {}

because q.B.m() does not override p.A.m(): p.A.m() is package-private and q.B is in another package. In v2, p.A.m() remains an abstract obligation that external subclasses of q.B cannot implement.

Roseau currently treats same-erasure subtype methods as overriding without checking package-private visibility, but in this case q.B.m() does not override p.A.m().