Form Class translation extraction

**Symfony version(s) affected**: 4.4.2 and 5.0.2

**Description**  
Labels defined in a Form Class are not extracted to translation file

**How to reproduce**  
Download symfony/demo, remove translation files, run

```bash
translation:update --force de
```

search for translation "label.tags". It is defined in Form/PostType as label, but it is missing now in the translation file.
