Idea: ContainerBuilder: avoid fopen if file is in opcode cache

Bored at school I got an idea to avoid touching disk for PHP 5.5.11+. It relies on undocumented function opcache_is_script_cached (http://stackoverflow.com/questions/23229849/opcache-is-script-cached-function-in-php5-5-11).

Not sure how to solve the atomicity. We may use `@include` and check return value for `FALSE`.