Adopt cargo's new config-include feature #2617
Description
Cargo recently stabilized the ability to include tomls in a .config/cargo.toml (rust-lang/cargo#16284, https://blog.rust-lang.org/inside-rust/2026/01/07/this-development-cycle-in-cargo-1.93/#config-include). We should use this to deduplicate entries in our .config/cargo.tomls between OSS and internal repos.