Allow choosing multiple package sources

[JokerGermany]

nala is using multiple sources when they are in the file.
https://gitlab.com/volian/nala#parallel-downloads

Nala will download 3 packages at a time per unique mirror in your sources.list file.
This constraint is to limit how hard Nala hits mirrors.
Opening multiple connections to the same mirror is great for speeding up downloading many small packages.
Additionally we alternate downloads between the available mirrors to improve download speeds even further.
If a mirror fails for whatever reason, we just try the next until all defined mirrors are exhausted.

Would be nice to be able to choose several package sources or at least warn that they are overwritten.