lib: create 'my' extensions

Inspired by this [1] repo, I want to add some functions to `lib.my`. I
would have liked to make it work without having to use an overlay, but I
did not manage to do it...

[1]: https://github.com/hlissner/dotfiles

lib/modules.nix

@@ -0,0 +1,23 @@
+{ self, lib, ... }:
+let
+  inherit (builtins) readDir pathExists;
+  inherit (lib) hasPrefix hasSuffix nameValuePair removeSuffix;
+  inherit (self.attrs) mapFilterAttrs;
+in
+{
+  mapModules = dir: fn:
+    mapFilterAttrs
+      (n: v:
+        v != null &&
+        !(hasPrefix "_" n))
+      (n: v:
+        let path = "${toString dir}/${n}"; in
+        if v == "directory" && pathExists "${path}/default.nix"
+        then nameValuePair n (fn path)
+        else if v == "regular" &&
+          n != "default.nix" &&
+          hasSuffix ".nix" n
+        then nameValuePair (removeSuffix ".nix" n) (fn path)
+        else nameValuePair "" null)
+      (readDir dir);
+}