You cannot do what you are trying to do, at least not easily. It's a one-way dependency from the plugins, which are compiled first. The only way to do it is to create a call-back mechanism in your plugin that you can register against. This way, your app will register itself with the plugin and plugin will call the classes when needed.
Think of plugins and their dependencies as jars - you cannot download a jar from some library and expect it to call your code, unless it's designed that way.
If you have helper classes, assuming they don't depend on anything in your main app, I'd suggest you move them from main app to a separate plugin that your local plugin depends on. This way both your main app and your local plugin depend on the helper classes. Or just move the helper classes to the plugin in the first place - they will then be available to both the plugin and main app.