add check for missing required dependencies

[maglnet]

Q	A
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues

Within a project of mine, I updated all dependencies and after that I got an error because of a missing Class.
This happens because the symfony/validator package is not defined as a dependency (only as a dev-dependency) but it seems to be required not only in tests.

This PR does the following:

• check if dependencies used within the autoload section are defined as a requirement
• add missing requirements to composer.json

Error: Class "Symfony\Component\Validator\Constraint" not found
/project-dir/vendor/nelmio/api-doc-bundle/src/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php:217
/project-dir/vendor/nelmio/api-doc-bundle/src/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php:196
/project-dir/vendor/nelmio/api-doc-bundle/src/ModelDescriber/Annotations/SymfonyConstraintAnnotationReader.php:59
/project-dir/vendor/nelmio/api-doc-bundle/src/ModelDescriber/Annotations/AnnotationsReader.php:71
/project-dir/vendor/nelmio/api-doc-bundle/src/ModelDescriber/ObjectModelDescriber.php:174
/project-dir/vendor/nelmio/api-doc-bundle/src/Model/ModelRegistry.php:115
/project-dir/vendor/nelmio/api-doc-bundle/src/ApiDocGenerator.php:135
/project-dir/vendor/nelmio/api-doc-bundle/src/Render/RenderOpenApi.php:85
/project-dir/tests/src/ValidOpenAPISyntaxTest.php:32

[DjordyKoert]

The error you encountered should be fixed with #2276 :)

[maglnet]

Thanks @DjordyKoert .
So I'll close this as obsolete.