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)

compiler/testData/asJava/nullabilityAnnotations/Primitives.java

@@ -1,4 +1,4 @@
-public interface Primitives extends jet.JetObject {
+public interface Primitives extends kotlin.jvm.internal.KObject {
     int $$int /* Real name is 'int' */(@jet.runtime.typeinfo.JetValueParameter(name = "x") int x);
 
     @org.jetbrains.annotations.Nullable
@@ -40,4 +40,4 @@ public interface Primitives extends jet.JetObject {
     float getFloat();
 
     double getDouble();
-}
+}