parts/lib: properly extend nixpkgs library

parts/lib/default.nix

@@ -1,16 +1,46 @@
 {inputs, ...}: let
-  callLibs = path:
-    import path {
-      inherit inputs;
-      lib = extendedLib;
+  inherit (inputs.nixpkgs) lib;
+
+  # Define the base library to be extended. Not only will this be an extension
+  # of the nixpkgs library, but it will also be extensible the way nixpkgs lib
+  # is in case the extended library
+  #  1. needs to be extended internally
+  #  2. needs to be extended during outside consumption
+  # Additionally, .extend can be used to extend the lib with
+  # custom, composed functions.
+  lib0 = self: let
+    callLibs = file:
+      import file {
+        inherit inputs;
+        lib = self;
+      };
+  in {
+    extendedLib = {
+      builders = callLibs ./builders.nix;
     };
-  extendedLib = inputs.nixpkgs.lib.extend {
-    builders = callLibs ./builders.nix;
+
+    # This makes mkSysytem available *in addition* to `lib.extendedLib.builders.mkSystem`.
+    # The syntax is a matter of preference, but it is good to make sure all custom attribute
+    # sets (e.g., builders) are defined and taken from a separate attrset (extendedLib) to make
+    # absolutely sure we *never* conflict with nixpkgs. Likewise, the function names inherited
+    # here should also be different from ones available under `lib` by default, i.e., you cannot
+    # re-define functions.
+    inherit (self.extendedLib.builders) mkSystem;
   };
+
+  # This can be used with `.extend` to extend the extended library.
+  extendedLib = lib.makeExtensible lib0;
 in {
-  # perSystem = {
-  #   _module.args.lib = lib;
-  # };
+  # Set the `lib` argument in `perSystem`, for example:
+  # ```
+  # perSystem = {pkgs, lib, ...}: { ...}
+  # ````
+  perSystem._module.args.lib = extendedLib;
+
+  # Prepare `lib` for external usage. This means it can be consumed
+  # from inputs.<name of this flake as an input>` by other flakes.
+  # `self` can also be used to access the custom lib as long as
+  # `flake.lib` is set.
   flake = {
     lib = extendedLib;
     _module.args.lib = extendedLib;