[Pre-RFC] DNS domains as package namespaces

While it's not likely that the domains will lapse, it is definitely something that needs to be considered as an inevitability for something .

I think the exact details of the policy for guarding against lapsed domains for major projects would make for a great section in the eventual RFC, if this thread ever gets to the point where an RFC gets written. It may involve some combination of associating namespaces with a set of crates.io users / teams allowed to push to those namespaces, a non-GitHub source of truth for crates.io teams, and/or requiring 2FA for namespaced crates.

However, right now, this thread is a pre-RFC because I don't know whether the Cargo or crates.io teams are OK with any form of namespaces. Any brain power spent pondering what to do if tokio.rs expires is wasted if I can't convince the relevant authorities that the packages xml and example.com/xml should be allowed to coexist.