Port MigrateSourceXML to Drupal 8 migrate source plugin

New functionality (primarily allowing certain elements that have been registered ahead of time to be referenced in xpath-like epxressions as parents of the context node) and lots of Drupal CS revisions.

This code and my work on a wordpress_migrate for D8 have now been used as parts of a successful site migration.

Let's see what happens with tests.

I need to post/update the wordpress_migrate for D8 module I used, this is rather opaque without it...

Ok that's at https://github.com/mbaynton/wordpress_migrate

@mbaynton - Just wanted to say I don't mean to ignore you, the XML source plugin and wordpress_migrate are important for D8 - at the moment, though, the top priorities are the core migration framework and the D6/D7 upgrade paths, so I haven't had time to give this the detailed review it deserves. That will come, but not immediately.

Thanks for your work on this.

@mikeryan Sure. Thanks for the update.

Finally ready to dig into this by using it in #2609284: Add XML migrations to wine example.

But, much like different field types like text and link have different modules, I'd like to see this go into a separate module/project. @mbaynton, unless you'd like to create the d.o project migrate_source_xml for this, I'll go ahead and do it myself - I'd be happy to add you as a co-maintainer with your permission.

Would this be a sub-module of migrate_plus, similar to CSV or separate? And if separate, could we get CSV split out as well?

Yes, separate - and I would like to separate migrate_source_csv out too. I'm trying to think about the logistics of that for existing migrate_source_csv users - removing it from migrate_plus at the same time will obviously break them, but keeping it there and deprecated for a transition period would be problematic in terms of having the same module name twice...

@mikeryan yes please go ahead with the d.o project. I'm on vacation so would be awhile for me to do it. You can add me as co-maintainer. Thanks!

Done! You are the co-maintainer of https://www.drupal.org/project/migrate_source_xml, and I've committed your initial patch.