K2: introduce platform-specific visibility checks for overrides + tests

Related to KT-53197

compiler/testData/diagnostics/tests/visibility/notOverridingPackagePrivate.kt

@@ -0,0 +1,23 @@
+// FILE: base/Base.java
+package base;
+
+public abstract class Base {
+    public void foo() {
+        packagePrivateFoo();
+    }
+
+    /* package-private */ void packagePrivateFoo() {};
+}
+
+// FILE: Impl.kt
+package impl
+import base.*
+
+class Impl : Base() {
+    fun packagePrivateFoo() { /*not an override*/ }
+}
+
+fun foo() {
+    Impl().foo()
+    Impl().packagePrivateFoo()
+}