use overlays argument of <nixpkgs> instead of custom code

The custom code returns the overlay instead of all packages but we
can sort-of fake it by filtering the resulting package set.

The custom code doesn't add the new packages to pkgsBuildBuild and it
doesn't create spliced packages for them so it won't work for cross
compiling.

release.nix

 let
-  mkPkgsCross = system: binaryCacheSystem: let
+  nixpkgs = import <nixpkgs>;
+  overlay = import ./overlay.nix;
+  addOverlay = overlay: extraArgs: extraArgs // { overlays = [ overlay ] ++ (extraArgs.overlays or []); };
+
+  mkPkgsCross = system: binaryCacheSystem: extraArgs: let
     # see https://nixos.wiki/wiki/Cross_Compiling#Lazy_cross-compiling
-    pkgsFromCache = import <nixpkgs> {
+    pkgsFromCache = nixpkgs ({
       config = {};
-      overlays = [];
+      overlays = [ ];
       system = binaryCacheSystem;
+    } // extraArgs);
+
+    getPkgsFromCache = pkgs: { inherit (pkgs) ncurses python3; };
+    takeSomePkgsFromCache = self: super: getPkgsFromCache pkgsFromCache // {
+      pkgsBuildHost = super.pkgsBuildHost // getPkgsFromCache pkgsFromCache;
     };
+    maybeTakeSomePkgsFromCache = if binaryCacheSystem == null then x: x else addOverlay takeSomePkgsFromCache;
 
-    pkgsCross = import <nixpkgs> {
-      overlays = if binaryCacheSystem == null
-        then []
-        else [(self: super: {
-            inherit (pkgsFromCache)
-              TODO;
-          })];
+    pkgsCross = nixpkgs ({
       crossSystem.config = system;
-    };
+    } // maybeTakeSomePkgsFromCache extraArgs);
   in pkgsCross;
 in
 { crossAarch64 ? false
 , crossSystem ? if crossAarch64 then "aarch64-unknown-linux-gnu" else null
 , crossBinaryCacheSystem ? if crossAarch64 then "aarch64-linux" else null
 , system ? builtins.currentSystem
+, extraArgs ? {}
 , pkgs ? if crossSystem == null
-  then import <nixpkgs> { inherit system; }
-  else mkPkgsCross crossSystem crossBinaryCacheSystem
+  then import <nixpkgs> ({ inherit system; } // (addOverlay overlay extraArgs))
+  else mkPkgsCross crossSystem crossBinaryCacheSystem (addOverlay overlay extraArgs)
 }:
-let
-  # copied from https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/splice.nix
-  splicedPackages = pkgs: pkgs.splicePackages {
-    inherit (pkgs)
-      pkgsBuildBuild pkgsBuildHost pkgsBuildTarget
-      pkgsHostHost pkgsHostTarget
-      pkgsTargetTarget
-      ;
-  } // {
-    # These should never be spliced under any circumstances
-    inherit (pkgs)
-      pkgsBuildBuild pkgsBuildHost pkgsBuildTarget
-      pkgsHostHost pkgsHostTarget
-      pkgsTargetTarget
-      buildPackages pkgs targetPackages
-      ;
-    inherit (pkgs.stdenv) buildPlatform targetPlatform hostPlatform;
-  };
-
-  super = pkgs // { callPackage = pkgs.lib.callPackageWith (splicedPackages self); };
-  self = super // locals;
-  locals = import ./overlay.nix self super;
-in locals
+pkgs.lib.attrsets.mapAttrs (name: _: pkgs."${name}") (overlay {} {})