Updating eager load to use zeitwerk

[jonhainstock]

Rails.application.eager_load! wasn't mapping automatically so I updated this to use Zeitwerk. Check this link for more information.

[joevandyk]

can this get merged?

[fxn]

This engine is eager loading because it needs to have all contentful models in memory, but forcing all loaders in memory to eager load is unnecessary (I assume all models are in the Rails application).

That initializer should need to be moved to a to_prepare block as explained in #61.