Currently we define Descriptor in specs-go/descriptor.go, but its media type in specs-go/v1/mediatype.go as application/vnd.oci.descriptor.v1+json. It seems like we should decide whether the type is being versioned or not.

• If it is being versioned:
  • shift Descriptor to specs-go/v1/descriptor.go
• If it is not being versioned (because we are committing to eternal forward compat):
  • shift MediaTypeDescriptor to specs-go/mediatype.go, and
  • rename the media type to application/vnd.oci.descriptor+json

We have the same inconsistency for layers, where the media types are defined in specs-go/v1/mediatype.go but lack a .v1 in the type string.