Align with rules_img layer file manifests

[peakschris]

rules_img is emerging as a new OCI image ruleset with many improved characteristics.

if rules_pkg could align with rules_img over direct image creation from pkg_files/filegroup/etc manifests this would make migration to rules_img much easier without the need for intermediate tar files.

There are details and a proposal here:
bazel-contrib/rules_img#74

@alexeagle @malt3

[tonyaiuto]

I'm on board with working towards a merged solution as outlined in Proposal for a "What-Goes-Where" Provider
.

My preference would be option 2, the starlark providers, but we will figure it out.