[Form] Composer dependencies

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Right now, launching tests from the Form component's directory doesn't work, due to missing dev dependencies. This should be merged back into master afterwards.

Also fixed a problem related to a bad method signature in ```Symfony\Component\Form\Tests\Extension\DataCollector\FormDataExtractorTest_SimpleValueExporter```