Fix errors baking tree models.

When generating code for tree models, bake incorrectly assumed that the
parent/child associations had separate tables. Reloading the registry by
itself should work, however, composer caches that files do not exist and
requires us to manually include the newly generated class.

Refs #4914
Merge pull request #4928 from cakephp/issue-4914

Fix errors baking tree models.