flake.nix: modularize (pt 1)

flake/legacyPackages.nix

@@ -0,0 +1,19 @@
+{inputs, ...}: {
+  perSystem = {
+    system,
+    inputs',
+    ...
+  }: {
+    legacyPackages = import inputs.nixpkgs {
+      inherit system;
+      overlays = [
+        inputs.tidalcycles.overlays.default
+        inputs.self.overlays.default
+        (_: _: {
+          rnix-lsp = inputs'.rnix-lsp.defaultPackage;
+          nil = inputs'.nil.packages.default;
+        })
+      ];
+    };
+  };
+}