Add a Save Source To Files feature for Migrate Source UI.

Problem/Motivation

Sometimes, when a User / Admin uploads a file via Migrate Source UI, you'd want to save the file at the location provided in the Migration Source Path.

For example, if the migration file's source contains :

source :
  plugin: csv
  path: sites/default/files/my-file.csv

The file uploaded will overwrite my-file.csv.

Proposed resolution

Add a checkbox in Migrate Source UI to upload the file or not.
Add in the form validation a file-move to the new path.

Here's an example image :

Remaining tasks

1. Review the code. There's bound to be some roadblock I didn't think about when writting the code.
2. Standardize the code : The code I've written is not compliant with Drupal Standards. I've also refactored the Form class a bit.
3. Write some tests ?