Move debug commands to debug namespace

Instead of having multiple namespace 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.