Allow the compiled container to have a namespace

If I do:

```
    $builder->enableCompilation(__DIR__ . '/cache', '\DrupalCodeBuilder\CompiledContainer');
```

I get:

```
  [InvalidArgumentException]
  The container cannot be compiled: `\DrupalCodeBuilder\CompiledContainer` is not a valid PHP class name
```

It should be possible for the container class to be namespaced.