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/Synthetic.java

@@ -1,14 +1,14 @@
-public final class Synthetic implements jet.JetObject {
+public final class Synthetic implements kotlin.jvm.internal.KObject {
     private final void foo() { /* compiled code */ }
 
     @org.jetbrains.annotations.NotNull
     public Synthetic() { /* compiled code */ }
 
-    public final class Inner implements jet.JetObject {
+    public final class Inner implements kotlin.jvm.internal.KObject {
         @org.jetbrains.annotations.NotNull
         public final void test() { /* compiled code */ }
 
         @org.jetbrains.annotations.NotNull
         public Inner() { /* compiled code */ }
     }
-}
+}