Islandora_7x_migrate: Make source PID available to default (MODS) row type.

[alxp]

PR: Islandora-Devops/migrate_7x_claw#31

We are creating a migration that includes paragraphs types in the destination content model.

Reference documentation for migrating in to paragraphs is here: https://www.fourkitchens.com/blog/development/migrating-paragraphs-drupal-8/

What it requires is to have dependent migrations that import in to the paragraph directly, then the node migration specifies those as dependencies. However, to match up imported paragraphs with their parents, the paragraph being migrated needs to have an identifier in common with the parent object.

For Fedora objects, this can be the source PID. However, when using a row type to import XML datastreams, the PID is not always available in the XML to find via Path.

I've added a step in the AuthenticatedXml plugin that adds PID to the item being imported so long as that field isn't already part of the import.