A Rust data model of the [APT External Dependency Solver Protocol](https://salsa.debian.org/apt-team/apt/-/blob/a8367745eac915281cc2b9fb98813e9225d1e55c/doc/external-dependency-solver-protocol.md).
Useful for writing custom dependency solvers for [APT](https://en.wikipedia.org/wiki/APT_(software)) in Rust.