default.nix: init

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.

default.nix

@@ -0,0 +1,22 @@
+let
+  lockFile = builtins.fromJSON (builtins.readFile ./flake.lock);
+  flake-compat-node = lockFile.nodes.${lockFile.nodes.root.inputs.flake-compat};
+  flake-compat = builtins.fetchTarball {
+    inherit (flake-compat-node.locked) url;
+    sha256 = flake-compat-node.locked.narHash;
+  };
+
+  flake' = (
+    import flake-compat {
+      src = ./.;
+    }
+  );
+  flake = flake'.defaultNix;
+in
+{
+  useFlake ? true,
+  nixpkgs ? if useFlake then flake.inputs.nixpkgs else <nixpkgs>,
+  pkgs ? import nixpkgs { },
+}:
+
+pkgs.callPackage ./scope.nix { }