[feature request] support `latest` switch for option -p

[ohad]

We can currently specify specific package collections using the -p option to pack. It would be nice to also support latest and fetch-latest as options, as we do for pack.toml files.

It might not be a huge thing to change, so even if you don't have the time to do it, feel free to sketch what needs changing and where. It might make a nice first-issue.