Switch to cargo-util-schemas structs

[markdingram]

https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#internal-1

New member crate cargo-util-schemas! This contains low-level Cargo schema types, focusing on serde and FromStr for use in reading files and parsing command-lines. Any logic for getting final semantics from these will likely need other tools to process, like cargo metadata. The crates.io publish of this crate is the same as other members crates. It follows Rust's 6-week release process. rust-lang/cargo#13178 rust-lang/cargo#13185 rust-lang/cargo#13186 rust-lang/cargo#13209 rust-lang/cargo#13267