oci: split up monolithic UnpackManifest

[cyphar]

• [ ] Un-merge the monolithic UnpackManifest function. We should have separate functions for unpacking a rootfs and generating the config.json (which we already have but this monolithic function does some stuff we should probably move elsewhere -- callers should be aware of the distinction between the two steps).
  • [ ] Make the rootfs == "" case work for more images with oci/spec/convert.