[VariationBundle] [ProductBundle] move 'sylius.validator.variant.unique' service definition

From VariationBundle to ProductBundle, to allow standalone usage of VariationBundle.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1449
| License       | MIT
| Doc PR        | --