Seal XmlCodec interface to deal with XmlCodecFactory warnings

XmlCodec is leaked from XmlCodecFactory, which is a public class. Prior to Java 15 we have had no way of restricting valid implementations, hence we kept the interface as package-internal. This leads to JPMS warnings about XmlCodec not being visible to clients.

With Java 17 we can happily expose the interface and still ensure there are no surprises.