add core configuration

modules/common/default.nix

@@ -0,0 +1,17 @@
+{lib, ...}: let
+  # recursively construct attr-set of a directory
+  getDir = dir:
+    lib.mapAttrs (file: type:
+      if type == "directory"
+      then getDir "${dir}/${file}"
+      else type) (
+      builtins.readDir dir
+    );
+  files = dir: lib.collect lib.isString (lib.mapAttrsRecursive (path: type: lib.concatStringsSep "/" path) (getDir dir));
+  validFiles = dir:
+    map (file: ./. + "/${file}") (
+      lib.filter (file: lib.hasSuffix ".nix" file && file != "default.nix") (files dir)
+    );
+in {
+  imports = validFiles ./.;
+}