[FrameworkBundle] [TwigBundle] Move debug commands to debug namespace

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

Instead of having several namespaces with only one debug command (container:debug, event-dispatcher:debug), move all these debug commands to a new **debug** namespace.

Related to https://github.com/symfony/symfony/pull/10388#issuecomment-49007419

I don't how to tag these aliases as deprecated as there are only here for backward compatibility.
The renaming should also be done in the Swiftmailer Bundle.