[Serialization] Reorganize module structure

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

@@ -0,0 +1,10 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE, -OPT_IN_USAGE
+// SKIP_TXT
+
+// FILE: test.kt
+import kotlinx.serialization.*
+
+class Foo(i: Int, val j: Int)
+
+<!EXTERNAL_CLASS_NOT_SERIALIZABLE!>@Serializer(forClass = Foo::class)<!>
+object ExternalSerializer