enable import of images from tar files

[deitch]

- What I did

• Simplified how cache.ImageLoad() works
• used it to create linuxkit cache import
• Removed accidental adding of too many manifests to cache index.json because of how it processed ImageLoad()

- How I did it

Lots of hard work.

- How to verify it

CI tests most of this. I also ran manual tests on a clean cache and on a populated cache

- Description for the changelog

Simplified ImageLoad, support for importing to cache