Fix autoloading

reverts 6151c2e8b1ad04e3a9cb60fff6b93c21a316e990

Since Trailblazer::Operation was previously defined as a Class, it cannot be redefined as a module, or you get "Operation is not a module (TypeError)".
That doesn't work because you define the constant on the top-level, which is wrong. It must be as follows.

```ruby
Trailblazer::Operation.class_eval do
  autoload ...
```