However, the main thrust of Chris's reply is correct.
There is no way to include a plugin in an spk and there is never likely to be.
If you wish to use a plugin in code published publicly, it would be helpful to include a comment at the top of your main code file, explaining exactly how to get the plugin.
Personally, I already had the GalleryPick plugin installed, though I don't remember ever testing it. I really am getting old.