[DependencyInjection] Fixed support for backslashes in service ids.

This change is for consistency with camelize() which is used in ProxyDumper and PhpDumper.
bug #9834 [DependencyInjection] Fixed support for backslashes in service ids. (jakzal)

This PR was merged into the 2.3 branch.

Discussion
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[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`?

Commits
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c6f210b [DependencyInjection] Fixed support for backslashes in service ids.