eager_autoload needs to be triggered for nested constants.

When `ActiveRecord::Associations.eager_load!` is called, it eager loads
the `Preloader` but does not trigger a `eager_load!` on the `Preloader`
itself. A eager loaded component is responsible to forward the
`eager_load!` call to nested constants that make use of `eager_autoload`
as well.

This will also resolve the occasional `NameError` that we've observed in
production:

```
NameError: uninitialized constant ActiveRecord::Associations::Preloader::BelongsTo
```

activerecord/lib/active_record/associations.rb

@@ -224,6 +224,11 @@ module ActiveRecord
       autoload :AliasTracker
     end
 
+    def self.eager_load!
+      super
+      Preloader.eager_load!
+    end
+
     # Returns the association instance for the given name, instantiating it if it doesn't already exist
     def association(name) #:nodoc:
       association = association_instance_get(name)