The `extends` field should support referencing OADs by their `$self`

[handrews]

See OAI/Arazzo-Specification#451 for an analogous requirement to support their Source Description Object's url field behaving the same way. It involves language about parsing potential target OAD documents supplied to the implementation for their $self field before deciding that no OAD document matches the URI.

IIRC, Overlay documents are never a target of any sort of references, so they don't really need their own $self at this time (although it might be nice for all three specs to support it, which would future-proof Overlays if they ever do need to be referenced — the above PR adds it for Arazzo).

[handrews]

Now that I think of it, the manifest idea in issue #291 could be a use case for Overlay having its own $self, assuming URLs/URIs are used to locate/identify things in the manifest. In particular, if a manifest uses a $self-respecting (hah!) URI for OAD and Arazzo documents, then we really should support such usage with Overlay documents as well.