Empty platform structs were already supported after splitting OCI Image
Index out from Docker Manifest List.
Signed-off-by: Bracken Dawson <abdawson@gmail.com>
Move implementation of the index from the manifestlist package to the ocischema package so that other modules making empty imports support the manifest types their authors would expect. This is a breaking change to distribution as a library but not the registry.
As OCI 1.0 released the manifest and index together, that is a good package from which to initialise both manifests. The docker manifest and manifest list remain in separate packages because one was released later.
The image index and manifest list still share common code in many functions not intended for import by other modules.
Signed-off-by: Bracken Dawson <abdawson@gmail.com>
