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.

flake.lock

@@ -1,5 +1,19 @@
 {
   "nodes": {
+    "flake-compat": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1751685974,
+        "narHash": "sha256-NKw96t+BgHIYzHUjkTK95FqYRVKB8DHpVhefWSz/kTw=",
+        "rev": "549f2762aebeff29a2e5ece7a7dc0f955281a1d1",
+        "type": "tarball",
+        "url": "https://git.lix.systems/api/v1/repos/lix-project/flake-compat/archive/549f2762aebeff29a2e5ece7a7dc0f955281a1d1.tar.gz?rev=549f2762aebeff29a2e5ece7a7dc0f955281a1d1"
+      },
+      "original": {
+        "type": "tarball",
+        "url": "https://git.lix.systems/lix-project/flake-compat/archive/main.tar.gz"
+      }
+    },
     "flake-parts": {
       "inputs": {
         "nixpkgs-lib": [
@@ -38,6 +52,7 @@
     },
     "root": {
       "inputs": {
+        "flake-compat": "flake-compat",
         "flake-parts": "flake-parts",
         "nixpkgs": "nixpkgs"
       }