Fix isJvmAccessible to handle nested protected Java classes

[tanishiking]

Fixes #24507

The previous implementation of isJvmAccessible failed to
recognize protected Java nested classes as accessible.
And it causes compilation errors when extending them from subclasses.

Previous implementation:

def isJvmAccessible(cls: Symbol): Boolean =
  !cls.is(Flags.JavaDefined) || {
    val boundary = cls.accessBoundary(cls.owner)(using preErasureCtx)
    (boundary eq defn.RootClass) ||
    (ctx.owner.enclosingPackageClass eq boundary)
  }

For protected nested classes like B below, the access boundary is the enclosing class (A in this case), not a package a or root.
However, B should be accessible through a public class A.

package a;

public class A {
  protected class B {}
}

This commit replace the manual boundary checks with cls.isAccessibleFrom, which properly handles all Java access modifiers.
This aligns with the Scala 2 implementation which uses the
equivalent context.isAccessible(cls, cls.owner.thisType).
https://github.com/scala/scala/blob/efb71845113639bd1da231c917e18af33519fc07/src/compiler/scala/tools/nsc/transform/Erasure.scala#L1451-L1455

For example,

• cls will be protected class B
• cls.owner.thisType will be public class A
• And, cls.isAccessibleFrom(cls.owner.thisType) is true.

On the other hand, isJvmAccessible returns false for Java-defined package protected class case like java-package-protected case.
https://github.com/tanishiking/scala3/tree/b2850be4c79996d8ebd9afaf36ed12f46febb0d1/tests/run/java-package-protected

In this case,

• cls = class B (package protected under a)
• cls.owner.thisType = package a
• And, cls.isAccessibleFrom(cls.owner.thisType) is false.

because B is package protected and, we cannot access B through a.B from another package. In this case, cast to A happens.

---

Note that, this regression starts from e7d479f but the true commit is d3ce551
but the reason why we stopped using isAccessibleFrom seems to be an optimization and the tests/pos/trait-access still compiles with this change.

[tanishiking]

@lrytz Do you mind taking a look? 🙏

[lrytz]

LGTM, thank you! But could you add the test from scala/scala#6023?

[lrytz]

Oh, also I think it would be better to make i24507 a run test.

[tanishiking]

Thanks @lrytz! Made i24507 run test 2a35ded
And, the test in scala/scala#6023 already ported to https://github.com/scala/scala3/tree/2fc524a43dca6f72e43a98cd11cb60570b1f163e/tests/run/java-package-protected as tests/run/java-package-protected by #21362 👍

[lrytz]

Thanks!