Tracking issue for exception about `extern crate` with sysroot crates

[carols10cents]

Rust tracking issue: rust-lang/rust#57288

When that is resolved, this paragraph should be updated/removed:

https://github.com/rust-lang-nursery/edition-guide/blame/master/src/rust-2018/module-system/path-clarity.md#L65-L87

[ehuss]

This has moved to https://github.com/rust-lang/edition-guide/blob/302a115e8f71876dfc884aebb0ca5ccb02b8a962/src/rust-2018/path-changes.md#an-exception in #232. Not sure how or if this exception will actually get fully resolved. I stabilized proc_macro when used with Cargo, but there are still some other cases.