FR: Support alternate URLs for package_index

[rohit-giridharan-ai]

We are using apt in apt.bzl to create a Debian package index. Our CI went down because the snapshot repository we were using, https://snapshot-cloudflare.debian.org, went down. https://snapshot.debian.org was still up - it would be nice to have the ability to add it as a mirror URL. I think it would require updating _fetch_package_index in package_index.bzl to support multiple URLs.

[BillyBlaze]

snapshot.debian.org was also not working for us (429 errors). We changed it to another mirror (utwente.nl) so it would be great if we could configure more beside debian.org.