nix: separate into multiple overlays and expose them from top-level

nix/overlays/default.nix

@@ -0,0 +1,5 @@
+{
+  overrides = import ./overrides.nix;
+  pkgs = import ./pkgs.nix;
+  posh = import ./posh.nix;
+}

nix/overlays/overrides.nix

@@ -0,0 +1,36 @@
+# This overlay is used for overriding upstream packages.
+
+self: super:
+
+let
+  nixpkgs-master = import <nixpkgs-master> { inherit (super) config; };
+
+in {
+  inherit nixpkgs-master;
+
+  # alacritty = nixpkgs-master.alacritty;
+  alacritty = super.stdenv.mkDerivation {
+    name = "alacritty-custom";
+    buildInputs = [ super.makeWrapper ];
+    phases = "installPhase";
+    installPhase = ''
+      makeWrapper ${self.nixpkgs-master.alacritty}/bin/alacritty $out/bin/alacritty \
+        --set-default WINIT_HIDPI_FACTOR 1.0
+    '';
+  };
+
+  roxterm = super.stdenv.mkDerivation {
+    name = "roxterm-custom";
+    buildInputs = [ super.makeWrapper ];
+    phases = "installPhase";
+    installPhase = ''
+      makeWrapper ${super.roxterm}/bin/roxterm $out/bin/roxterm \
+        --add-flags "--separate"
+    '';
+  };
+
+  # TODO: facetimehd is currfently broken (https://github.com/NixOS/nixpkgs/pull/72804)
+  facetimehd-firmware = super.hello;
+
+  qtile = self.nixpkgs-master.qtile;
+}

nix/overlays/pkgs.nix

@@ -0,0 +1,18 @@
+# This overlay includes all packages defined by the top-level default.nix. 
+# The code is copied from the NUR repository [0].
+#
+# [0]: https://github.com/nix-community/nur-packages-template/blob/2610a5b60bd926cea3e6395511da8f0d14c613b9/overlay.nix
+
+self: super:
+
+let
+
+  isReserved = n: n == "lib" || n == "overlays" || n == "modules";
+  nameValuePair = n: v: { name = n; value = v; };
+  nurAttrs = import ../pkgs { pkgs = super; };
+
+in
+  builtins.listToAttrs
+  (map (n: nameValuePair n nurAttrs.${n})
+  (builtins.filter (n: !isReserved n)
+  (builtins.attrNames nurAttrs)))

nix/overlays/posh.nix

@@ -0,0 +1,8 @@
+self: super:
+
+let
+  nixpkgs-master = import <nixpkgs-master> {};
+in {
+  inherit (nixpkgs-master) podman conmon slirp4netns;
+  posh = self.callPackage ../pkgs/posh.nix {};
+}