nixos/etc: keep directories in derivation name

This changes eg. etc-timesyncd.conf to etc-systemd-timesyncd.conf which
better reflects the actual location of the config file.

nixos/modules/system/etc/etc.nix

···
           config = {
             target = mkDefault name;
             source = mkIf (config.text != null) (
-              let name' = "etc-" + baseNameOf name;
+              let name' = "etc-" + lib.replaceStrings ["/"] ["-"] name;
               in mkDerivedConfig options.text (pkgs.writeText name')
             );
           };