Refactor behaviour triggered by a flag into separate classes.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes (refactoring)
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | we'll see :-)
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The ConfigCache class has two modes of behavior, toggled by the "debug" [flag](http://martinfowler.com/bliki/FlagArgument.html).

Refactoring that into two different classes may seem pointless or overkill at the moment, but gets more interesting once resource checking/validation gets more intricate. Peek at [this](https://github.com/webfactory/symfony/blob/2957057017259ff315c58e22637605963e303c11/src/Symfony/Component/Config/ResourceValidatingCache.php) if you're curious :-).

It's a pick off #7230.