Move lighttpd.nix to lighttpd/default.nix

So that we later can add sub-services for lighttpd without polluting the
web-servers/ directory.

modules/module-list.nix

@@ -185,7 +185,7 @@
   ./services/ttys/agetty.nix
   ./services/web-servers/apache-httpd/default.nix
   ./services/web-servers/jboss/default.nix
-  ./services/web-servers/lighttpd.nix
+  ./services/web-servers/lighttpd/default.nix
   ./services/web-servers/nginx/default.nix
   ./services/web-servers/tomcat.nix
   ./services/x11/desktop-managers/default.nix