Fix an issue where plugin associations would be mishandled.

When joining plugin models together, I ran into an issue when using
`hasMany('Plugin.Model')`. The SQL conditions would be incorrectly
generated as `Plugin.Model.foo_id` instead of `Model.foo_id`.
Fix failure caused by extra plugin classes being added.
Merge pull request #4176 from cakephp/plugin-association

3.0 Fix an issue where plugin associations would be mishandled.