Explicitly require all dependencies

[mbrodala]

Using https://packagist.org/packages/maglnet/composer-require-checker

$ composer-require-checker check composer.json
ComposerRequireChecker 4.7.1@e49c58b18fef21e37941a642c1a70d3962e86f28
The following 3 unknown symbols were found:
+-----------------------------------+--------------------+
| Unknown Symbol                    | Guessed Dependency |
+-----------------------------------+--------------------+
| Symfony\Component\Mailer\Envelope |                    |
| Symfony\Component\Mime\Email      |                    |
| Symfony\Component\Mime\RawMessage |                    |
+-----------------------------------+--------------------+