[DependencyInjection] Fixed support for backslashes in service ids.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9801
| License       | MIT
| Doc PR        | 

This change is needed for consistency with `camelize()` which is used in [`ProxyDumper`](https://github.com/symfony/symfony/blob/2.3/src/Symfony/Bridge/ProxyManager/LazyProxy/PhpDumper/ProxyDumper.php#L69) and [`PhpDumper`](https://github.com/symfony/DependencyInjection/blob/2.3/Dumper/PhpDumper.php#L1275). 

Either this PR needs to be merged for consistency or #9610 rolled back (if we don't want to support backslashes in service ids).

Anyone could tell me why we're not using the `camelize()` method internally in the `Container`?