Allow importing/exporting archives which doesn't have all the referenced
blobs. This allows to export/import an image with only some of the
platforms available locally while still persisting the full index.
> The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.
https://github.com/opencontainers/image-spec/blob/v1.0/image-layout.md#blobs
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
