Move jet.JetObject -> kotlin.jvm.internal.KObject

Don't use JetObject as a marker class in KotlinRuntimeLibraryUtil anymore,
since there are other classes for this purpose (KotlinPackage, Unit)

core/runtime.jvm/src/kotlin/jvm/internal/KObject.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package jet;
+package kotlin.jvm.internal;
 
-public interface JetObject {
+public interface KObject {
 }