Make it easy (and well performing) to skip certain subdirectories/globs within component dirs

[timriley]

Right now, the only way is to do this:

config.component_dirs.add "some_dir" do
  dir.auto_register = proc do |component|
    relative_path = component.file_path.relative_path_from(root).to_s
    !relative_path.start_with?("config#{File::PATH_SEPARATOR}")
  end
end

Which feels a bit too roundabout, and also imposes a cost on every auto-registration.

We may be able to take inspiration on how Zeitwerk does this.