Auto merge of #4571 - raytung:remove-root-from-cargo-lock, r=matklad

Remove root from cargo lock

Removing [root] section from Cargo.lock for #4563

Tests of interest has been updated accordingly, especially [tests/lockfile-compat.rs#oldest_lockfile_still_works](https://github.com/rust-lang/cargo/compare/master...raytung:remove-root-from-cargo-lock?expand=1#diff-48866d1891f97cb799dd0ca7148d1eefR10)