"How to import Products from a XML file" form extension does not work in 5.0

[Dnd-Smash]

In the "How to Customize Import / Export" guide, there is a sub-guide called "How to import Products from a XML file"

The configure the job section contains (a bit abruptly) some form extensions contents that do not work.

Since they were quite hard to debug, we used the csv_product_import_(show/edit).yml as starting point and refined them a bit to match this guide's need.