modules: allow root level nix files

More flexibility in allowing root level files to import, for those that
don't need nesting.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/modules.nix

@@ -77,9 +77,11 @@ let
     ++ (lib.concatMap
       (
         dir:
-        lib.pipe dir [
-          builtins.readDir
-          (lib.filterAttrs (_path: kind: kind == "directory"))
+        lib.pipe (builtins.readDir dir) [
+          (lib.filterAttrs (path: _kind: !lib.hasPrefix "_" path))
+          (lib.filterAttrs (
+            _path: kind: kind == "directory" || (kind == "regular" && lib.hasSuffix ".nix" _path)
+          ))
           (lib.mapAttrsToList (path: _kind: lib.path.append dir path))
         ]
       )