Change plugin files to be compatible with Drupal 8.9.x

[ecosty]

Hey Karen, thanks for this module which helped me a lot to understand how to migrate using the JSon API. However I had to change many things in order to make this work in Drupal 8.9.x. Please consider these PR so more people can use it and apply in their projects. Thanks!

BTW: I also used some information from here: https://understanddrupal.com/articles/migrating-files-and-images-drupal

Elias Rivera.