files: prefix relative path literal with './'

Fixes the `warn-short-path-literals` warning:

  warning: relative path literal 'lib/file-type.nix' should be
  prefixed with '.' for clarity

Nix recommends prefixing relative path literals with './' to
distinguish them from other path forms.

modules/files.nix

@@ -34,7 +34,7 @@ let
 
   inherit
     (
-      (import lib/file-type.nix {
+      (import ./lib/file-type.nix {
         inherit homeDirectory lib pkgs;
       })
     )