Support loading assets from Package.

You may or may not want to incorporate this pull request at this time. I won't be surprised if Xcode changes how assets in packages work. But this may be a good reference for others who want to load assets from a SPM bundle at this time. Once we get into the later betas for Xcode, perhaps we'll consider this pull request.

@McProb Thanks for submitting! I agree it would be best to wait until iOS 14 is finalized before merging this, I'll leave this up until then.

@McProb any idea if this behavior remained consistent in the iOS 14 GM? if so I can merge this

I'm not certain that this will stay working. I'll take a look in the next day or two and see if I can get the official way of doing it working.