feat(lib): add mapModules to map func over modules

lib/default.nix

@@ -13,4 +13,34 @@ lib.makeExtensible (self: rec {
       rev = input.shortRev or "dirty";
     in
     "unstable-${date}_${rev}";
+
+  mapModules = f: dir:
+    let
+      filter = name: type:
+        let
+          isPublic = !(lib.hasPrefix "_" name);
+          isSomething = type != null;
+          isModule =
+            let
+              path = "${toString dir}/${name}";
+              isDefault = type == "directory" && builtins.pathExists "${path}/default.nix";
+              isFile = type == "regular" && lib.hasSuffix ".nix" name && name != "default.nix";
+            in
+            isDefault || isFile;
+        in
+        isPublic && isSomething && isModule;
+
+      mkModule = name: _:
+        let
+          path = "${toString dir}/${name}";
+          normalizedName =
+            if name == "default.nix"
+            then name
+            else lib.removeSuffix ".nix" name;
+        in
+        lib.nameValuePair normalizedName (f path);
+    in
+    lib.mapAttrs' mkModule (lib.filterAttrs filter (builtins.readDir dir));
+
+  mapModules' = f: dir: lib.attrValues (mapModules f dir);
 })