Disambiguate provider of blocks (e.g., System blocks vs. Menu blocks)

This issue was resolved in the Menu Blocks module!

🐛 Suppress display of regular system menu blocks in Layout Builder RTBC

Closing as "works as designed"

Re-opening this issue. The duplicates are resolved in the Settings tray on an individual entity using Layout Builder, but the Layout Builder Restrictions list still duplicates them.

The merge request updates the logic to check if the Menu Block contrib module is active on the site. If so, it suppresses the (duplicative) core menus from the list. This is ready for review!