exports a constant for "oci-layout" but there seems to be no export anywhere in the package for the "blobs" or "index.json" which are the other two path names needed to implement https://github.com/opencontainers/image-spec/blob/main/image-layout.md. These strings are hard coded into outside implementations like https://github.com/moby/buildkit/blob/60d134bf7671e60bf2a2b9bc41a1d856e6ef396e/client/ociindex/ociindex.go#L16 and https://github.com/oras-project/oras-go/blob/v2.1.0/content/oci/oci.go#L41-L44

This issue is a request for some part of this repo to export those two constants so they can be used consistently across other projects relying on this spec.