[Intl] Added "internal" tag to all classes under Symfony\Component\Intl\ResourceBundle

We didn't have this tag yet when this component was first written. The code in that
namespace is only used for resource bundle generation and was never meant for public
use.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes?
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -