Support declaring nix system dependencies in Cargo.toml

[fogti]

e.g. when I have a -sys crate which links against C stuff using build.rs, and could reuse already installed stuff, it would be cool if I could declare a dependency in Cargo.toml (in the package.metadata section, e.g. [package.metadata.crate2nix]), and crate2nix should then (when giving a corresponding command line flag, e.g. should be opt-in) scan the whole dependency chain and add the appropriate dependencies.

[fogti]

alternative: allow specifying a map from "links" defined in Cargo.toml files to nixpkgs dependencies.

[danieldk]

I think it is nicer to add native dependencies (at least of widely-used crates) to the default crate overrides. Then everyone benefits from them across all projects.

[fogti]

Ah ok.