Move recipes from flake.nix:s `packages` to a `callPackage`-able scope.nix.
This way out-of-tree projects can reuse the `buildRustPackage`
definitions (e.g. the in-sync `cargoLock` hashes) without depending on
the unsupported flakes feature.
There is not so much of a difference in this project, but
it means adding this project to clan is one less flake input
and also more consistent with other projects in clan.
