replace autoloads with requires in sorcery.rb

As mentioned in https://github.com/NoamB/sorcery/issues/554 `#autoload` is deprecated, and not recommended.

I'm not really sure about this PR, as it may increase application boot time. I've checked it only on my sorcery-example-app and it the difference wasn't visible.

Some files loaded belong to `external` submodule (all the providers). In long term I'd love to extract this submodule to separate gem, this would reduce number of loaded files and remove a few dependencies from Sorcery. But that's a longer plan.