Fix incorrect type in ManifestRegistrar#finalize!

`#finalize!` is sending the basename directly to `#call`, which expects
a Dry::System::Identifier.

This means that if you configure a `registrations_dir`, putting any file
in that directory will lead to an exception during finalization.

This appears to have been introduced during #208

lib/dry/system/manifest_registrar.rb

@@ -27,7 +27,7 @@ def initialize(container)
       # @api private
       def finalize!
         ::Dir[registrations_dir.join(RB_GLOB)].sort.each do |file|
-          call(File.basename(file, RB_EXT))
+          call(Identifier.new(File.basename(file, RB_EXT)))
         end
       end