[Validator][Email] - When standalone "Fatal error: Class 'validator.email' not found"

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/silexphp/Silex/issues/932
| License       | MIT
| Doc PR        | no

When using the validator as an standalone component a bug was introduced by overriding the `validatedBy()` method in the `Email` constraint.
This PR removes the override and implements a way for the DI to get the right validator.

Another way would be modifying the definition of validator to change the alias instead of having the FQCN in the xml