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/Generic.java

@@ -1,4 +1,4 @@
-public interface Generic <N, NN> extends jet.JetObject {
+public interface Generic <N, NN> extends kotlin.jvm.internal.KObject {
     N a(@jet.runtime.typeinfo.JetValueParameter(name = "n") N n);
 
     @org.jetbrains.annotations.NotNull
@@ -9,4 +9,4 @@ public interface Generic <N, NN> extends jet.JetObject {
 
     @org.jetbrains.annotations.Nullable
     NN b1(@org.jetbrains.annotations.Nullable @jet.runtime.typeinfo.JetValueParameter(name = "nn", type = "?") NN nn);
-}
+}