flake: overlays: add hycov and default

Also changes how package are built. Now, `nixpkgs` is imported in
`withPkgsFor` using Hyprland's `hyprland-packages` overlay and this
flake's default. The flake's `packages.<system>` outputs are inherited
from `pkgs.hyprlandPlugins`. This produces the package output `hycov`,
but I have also added `default`.

This matches <hyprwm/hyprland-plugins#180>.

flake.nix

@@ -1,30 +1,50 @@
 {
-  description = "Hyprland Plugins";
+  description = "Hyprland Plugins (Hycov)";
 
   inputs.hyprland.url = "git+https://github.com/hyprwm/Hyprland?submodules=1";
 
   outputs =
     { self
     , hyprland
-    ,
     }:
     let
       inherit (hyprland.inputs) nixpkgs;
-      withPkgsFor = fn: nixpkgs.lib.genAttrs (builtins.attrNames hyprland.packages) (system: fn system nixpkgs.legacyPackages.${system});
+      inherit (nixpkgs) lib;
+      systems = lib.attrNames hyprland.packages;
+      withPkgsFor = fn: lib.genAttrs systems (system:
+        let
+          pkgs = import nixpkgs {
+            localSystem.system = system;
+            overlays = [
+              hyprland.overlays.hyprland-packages
+              self.overlays.default
+            ];
+          };
+        in
+          fn system pkgs);
     in
     {
-      packages = withPkgsFor (system: pkgs: {
-        hycov = pkgs.callPackage ./default.nix {
-          inherit (hyprland.packages.${system}) hyprland;
-          stdenv = pkgs.gcc13Stdenv;
+      overlays = {
+        default = self.overlays.hycov;
+        hycov = final: prev: {
+          hyprlandPlugins = prev.hyprlandPlugins or {} // {
+            hycov = final.callPackage ./default.nix {
+              stdenv = final.gcc13Stdenv;
+            };
+          };
         };
+      };
+
+      packages = withPkgsFor (system: pkgs: {
+        default = self.packages.${system}.hycov;
+        inherit (pkgs.hyprlandPlugins) hycov;
       });
 
       devShells = withPkgsFor (system: pkgs: {
         default = pkgs.mkShell.override { stdenv = pkgs.gcc13Stdenv; } {
           name = "hyprland-plugins";
-          buildInputs = [ hyprland.packages.${system}.hyprland ];
-          inputsFrom = [ hyprland.packages.${system}.hyprland ];
+          # buildInputs = [ pkgs.hyprland ];
+          inputsFrom = [ pkgs.hycov ];
         };
       });
     };