Breaking out from #53128

Currently, extern crate is still required for sysroot crates, like test, proc_macro, core or std. One needs to extern crate them for use in no_std crates unless they are implicitly imported (like std is without #![no_std], or core with it).

• proc_macro today is automatically imported by Cargo for proc-macro declared crates
• alloc and std need extern for no_std (or std crates)
• core is automatically extern'd in for all crates

This is the tracking issue for making them not require extern crate.