`packages` backup should be parallelized / multithreaded

`backup_packages()` should spin up a bunch of workers in a pool to actually do the backing up, instead of doing it one at a time.

It's a minor bug for me (since I rarely use this workflow) but it should get fixed. No reason it can't be fast.

This is a pointer to the current implementation: https://github.com/alichtman/shallow-backup/blob/main/shallow_backup/backup.py#L148-L233