Supervisor doesn't await a plugin preload request when a method on that plugin is called

[cool-ant]

Flow that works

• await new Supervisor()
• make call to a plugin

Flow that fails

• await new Supervisor()
• request preloading of a plugin
• immediately make call to that plugin

The method call fails, rather than Supervisor waiting for the plugin to load (as it does if preload is not called).

Supervisor should await any loading that's in progress if it's interfering with the current method call.