Fix Magento Product source plugin "all_sku"

So.. I am rolling up my sleeves finally :) and presume that maybe this is due to php8.2 on my end, and maybe php8 is just none-too-happy with things like:
```
$file_data['all_sku'][] = $row['sku'];
```
before `$file_data['all_sku']` even exists. Ok, I'll plug through and see where I get.

Just what is the right way to use this looks-pretty-really-useful example migration? Not enabling it, the Product.php plugin is missing. Enabling it, a whole bunch of example migrations get enabled (even without `./script/cp_migrations magento`)

Attached is a reworked `Product.php`. I don't know if I did it right, but the changes overcome the PHP fatal error in this issue queue when I run `drush ms`, and migrations are all listed out as expected now.

Please see my question above about how best to use this submodule in a real project. Is it just copy/paste? What all should we copy/paste?

@texas-bronius, thanks you for taking the time to fix this!

quietone → closed merge request !19

Automatically closed - issue fixed for 2 weeks with no activity.