feat: support for ref aliases when resolving components

[cmars]

[Backport of #161 into 3.x]

This adds minimal support for JSON References, section 4.1[0] in which a
$ref prefix can resolve through another $ref in the JSON document.

It is not a complete implementation of this specification, but enough of
one to be useful for resolving OpenAPI components. Limitations:

• Reference prefixes may only be defined as a component extension.
• A single substitution pass is performed on the document only once.
• Therefore, only a single level of indirection is supported.

Drive-bys:

• Replace the Localize implementation with openapi3/T.InternalizeRefs.