There isn’t any. This is intentional, because doing this would be a terrible idea.
Allowing people to refer to namespaces from within the source code would make it impossible to switch out unmaintained crates with maintained ones (who have been forked and live under a different organization) without having to touch every single source file.
That’s an atypical understanding of namespaces. Not that it is necessarily bad. I think the ability to swap out unmaintained packages for maintained packages has a certain appeal, but what you are proposing does run counter to what people will be used to. In particular it is not what the other main domain-based namespacing system does (the JVM ecosystem). It is also not how npm scopes work.
Including namespaces in how you refer to packages in code is the standard way to do things. Calling it a terrible idea out of hand is unjustifiably dismissive. The position that conflicts won’t happen much and when they do you can just use crate renaming is an interesting one. It has the benefit of keeping the language the same. I really like the idea of keeping the changes confined to the package manager.