Multi-crate workspace support

[srid]

rust-flake currently supports only Rust projects that have a single top-level crate. https://github.com/srid/nixci is a good example, which already uses rust-flake.

However, for Rust projects with multiple crates (such as https://github.com/juspay/superposition), we'll need to modify our API in some way to support building them, and pick a few as flake outputs (superposition outputs a single crate called superposition).

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Concretely, change this module sufficiently so that we can use it in https://github.com/juspay/superposition. Superposition currently uses crane directly. The idea is to supplant its rust.nix by using rust-flake, just as nixci does.