--add-to-scope should add to scope only predefined packages

[skelly37]

Describe the issue

• add somme attribute to jsons, like importable which would be a list of files to import to the scope
• document them
• make use of them while installing/removing packages
• check each sourced file if it's .nu, either before adding to the scope or when generating the repo cache (I'd say do the latter)

Argumentation

1. It would allow to self-host nupac as a way to install any files, like Firefox or Telegram
2. It would allow us to avoid unnecessary errors with multiple .nu files in the dir
3. It would prevent from adding invalid files to scope

Screenshots and other helpful media

No response

Additional context

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