Fix deprecation warnings

Author: zlw

spec/dummies/without-engine/dummy/config/initializers/system.rb

@@ -4,7 +4,7 @@
 
 Dry::Rails.container do
   config.component_dirs.add "lib" do |dir|
-    dir.default_namespace = "dummy"
+    dir.namespaces.add "dummy", key: nil
   end
 
   config.component_dirs.add "app/operations"

spec/integration/dry/rails/railtie/finalize_engine_spec.rb

@@ -12,7 +12,7 @@
 
     Dry::Rails::Engine.container(:super_engine) do
       config.component_dirs.add "app/forms" do |dir|
-        dir.default_namespace = "super_engine"
+        dir.namespaces.add "super_engine", key: nil
       end
     end