Move export for plugin declaration (#555)

Change the expected exported name of plugin declarations to be "plugin"
at the main entrypoint instead of the default export.  This makes the
plugin's code organisation more flexible especially when using CommonJS
JavaScript modules as they can export additional symbols from the
plugin declaration file.

packages/controller/src/BaseControllerPlugin.ts

@@ -13,8 +13,8 @@ import type HostConnection from "./HostConnection";
  * Controller plugins are subclasses of this class which get instantiated by
  * the controller on startup when the plugin is enabled in the config.
  * To be discovered the class must be exported under the name `ControllerPlugin`
- * in the module specified by the `controllerEntrypoint` in the plugin's info.js
- * file.
+ * in the module specified by the `controllerEntrypoint` in the plugin's
+ * `plugin` export.
  */
 export default class BaseControllerPlugin {
 	/**