Integrate DataNodeIdentifier with YangXPathExpression

YangXPathExpression is lacking the proper data reference type, i.e. we cannot express YANG's instance-identifier type in a reasonable form.

YANGTOOLS-878 proposes to address the translation need in terms of what is needed for NormalizedNodes, but there are two sides to that problem. And adoption of DataNodeIdentifier would help that effort, as NormalizedNodes can serve as backing store to that addressing mode, if there is a suitable translator orchestrating the two addressing modes.

As an example, search along data tree axis in a ContainerNode is union of its direct children and augmentations (and maybe choices, I always forget).