[Serialization] Reorganize module structure

plugins/kotlinx-serialization/testData/diagnostics/DuplicateSerialName.kt

@@ -0,0 +1,10 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
+// WITH_STDLIB
+// FILE: test.kt
+import kotlinx.serialization.*
+
+@Serializable
+open class Parent(open val arg: Int)
+
+<!DUPLICATE_SERIAL_NAME("arg")!>@Serializable<!>
+class Derived(override val arg: Int): Parent(arg)