feat(flake.nix): Add dmd, ldc and dub as Flake outputs

* Add `pkgs/default.nix` file that imports all packages
* Add `dmd`, `ldc` and `dub` to `perSystem.packages` section
* Use `flake-parts.flakeModules.easyOverlay` to derive an overlay

Result:

    > nix flake show

    ...
    ├───overlays
    │   └───default: Nixpkgs overlay
    └───packages
        ├───x86_64-linux
        │   ├───dmd: package 'dmd-2.098.0'
        │   ├───dub: package 'dub-1.27.0'
        │   └───ldc: package 'ldc-1.28.0'
        ...

flake.nix

@@ -19,6 +19,7 @@
  }:
  flake-parts.lib.mkFlake {inherit inputs;} {
  systems = ["x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin"];
+ imports = [./pkgs];
  perSystem = {pkgs, ...}: {
  devShells.default = import ./shell.nix {inherit pkgs;};
  };

pkgs/default.nix

@@ -0,0 +1,23 @@
+{inputs, ...}: {
+ imports = [inputs.flake-parts.flakeModules.easyOverlay];
+
+ perSystem = {
+ self',
+ pkgs,
+ ...
+ }: let
+ inherit (pkgs) callPackage;
+ darwinPkgs = {
+ inherit (pkgs.darwin.apple_sdk.frameworks) Foundation;
+ };
+ in {
+ overlayAttrs = self'.packages;
+ packages = {
+ dmd = callPackage ./dmd ({} // darwinPkgs);
+
+ ldc = callPackage ./ldc {};
+
+ dub = callPackage ./dub {};
+ };
+ };
+}