Support SNS indices in definition root nodes (config and content)

We currently do not allow SNS indices to appear in config or content definition root node paths, such as:

definitions:

  config:

    /rootChild/child[2]:
      jcr:primaryType: whatever

We have determined that this complicates our lives too much when trying to create consistent output via auto-export (REPO1709), so we want to relax this restriction and allow this to be considered valid. This will require us to do more sophisticated sorting of definition root paths, since a simple lexical sort will lead to incorrect results for e.g. [2] and [10]. We will need to parse the paths and do a more sophisticated mixed lexical/numeric sort.

If we're going to do this more complex type of path processing, we should also take the opportunity to update our API to use a Path/Name object representation instead of raw Strings for JCR paths.