[Serialization] Reorganize module structure

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

@@ -0,0 +1,18 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
+// WITH_STDLIB
+// FILE: test.kt
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+
+@Serializable(NopeNullableSerializer::class)
+class Nope {}
+
+class NopeNullableSerializer: KSerializer<Nope?> {
+    override val descriptor: SerialDescriptor get() = TODO()
+    override fun deserialize(decoder: Decoder): Nope? = TODO()
+    override fun serialize(encoder: Encoder, value: Nope?) = TODO()
+}
+
+@Serializable
+class Foo(val foo: <!SERIALIZER_NULLABILITY_INCOMPATIBLE("NopeNullableSerializer", "Nope")!>Nope<!>)