Fetch plugin commands from multiple runtimes

[FichteFoll]

With ST4000+ switching to a multi-runtime plugin environment, our method of fetching all plugin-defined commands is currently only able to fetch those of the same runtime. Since the other runtime is isolated, the only solution I can think of is a command-based communication between the main PD host and a shim package that only fetches the commands of the other runtime, comparable to what AutomaticPackageReloader does. (randy3k/AutomaticPackageReloader#37)

For now, PD and most other packages won't run on plugin_host-3.8 due Package Control not being updated to it and our reliance on dependencies, but we can still generate a package shim to fetch the Default commands.

[FichteFoll]

Will be a high priority once PC has proper support for the 3.8 host.