This is kind good reason to have optional namespaces in Cargo. The Rust team could have a "rust/" prefix or something with things that are considered to be a "standard library extension," but supports versioning and all that. Some notes about that namespace:

never imports libraries outside its namespace
releases align with Rust releases where relevant
all new stdlib packages go through this namespace first
releases follow the same standards as a change to the standard library (semver for breaking changes instead of editions, maintenance of old major versions according to stdlib policy)

This solves most of the problems a large standard library is intended to solve, while avoiding most of the downsides. Users only need to look in one namespace for "official" packages, so it's similar to looking at the standard library (official Rust docs could even include them).