Expose some parts in a Rust crate

[figsoda]

In a conversation in the Nixpkgs Architecture Team matrix, it was brought up that maybe the structure of name-based package directories can be exposed as an API.

My proposal is that maybe some constants and functions from check-by-name should be exposed in a Rust crate:
https://github.com/NixOS/nixpkgs/blob/7dc6783ecf8ae22c4a89a6ca4be74de238262045/pkgs/test/nixpkgs-check-by-name/src/structure.rs#L9-L10
https://github.com/NixOS/nixpkgs/blob/7dc6783ecf8ae22c4a89a6ca4be74de238262045/pkgs/test/nixpkgs-check-by-name/src/structure.rs#L28

cc @nixpkgs-architecture @nixpkgs-architecture/team

[figsoda]

since the nixpkgs-architecture ping didn't seem to work: @infinisil @roberth