improve packages overlay + add comments explaining

global-default.nix

@@ -5,9 +5,8 @@
   specialArgs,
   ...
 }: let
-packageFileNames = builtins.attrNames (builtins.readDir ./packages);
-packageNames = map (x: lib.removeSuffix ".nix" x) packageFileNames;
-packagesOverlay = (final: prev: lib.genAttrs packageNames (name: (prev.pkgs.callPackage ./packages/${name}.nix { })));
+packageNames = map (name: lib.removeSuffix ".nix" name) (builtins.attrNames (builtins.readDir ./packages)); # creates a list of files in ./packages, with their '.nix' extensions removed
+packagesOverlay = (final: prev: lib.genAttrs packageNames (name: (prev.pkgs.callPackage ./packages/${name}.nix { }))); # creates a function (overlay) that returns an attrset of names paired to 'callPackage' calls
 in {
   imports = [
     ./hosts/${specialArgs.hostname}/hardware-configuration.nix