[FIR] Change API for generating nested classes from plugins

`FirDeclarationGenerationExtension.generateClassLikeDeclaration` was split
  into two functions: one for generating top level classes, and one for
  nested classes. Such change reduces verbosity and error-proness of
  this extension and also allows to smoothly run plugins on local classes

^KT-55248 Fixed

plugins/kotlinx-serialization/testData/firMembers/serializerInLocalClass.kt

@@ -0,0 +1,20 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+
+fun local(): String {
+    @Serializable
+    data class Carrier(val i: Int)
+
+    return Carrier.serializer().descriptor.toString()
+}
+
+fun box(): String {
+    val expected = "local.Carrier(i: kotlin.Int)"
+    val actual = local()
+    if (expected != actual) {
+        return "Fail: $actual"
+    }
+    return "OK"
+}