Make scopes work with polymorphic relation directives

[spawnia]

• [x] Added or updated tests
• [ ] Documented user facing changes
• [ ] Updated CHANGELOG.md

Resolves https://github.com/nuwave/lighthouse/issues/2106

Changes

Allows using scopes in polymorphic relation directives and actually apply them to the relation.

• [ ] Clean up the abstraction for scopes
• [ ] Consider other polymorphic relation directives

Breaking changes

Polymorphic directives will change in definition and function, but the previous behaviour made little to no sense.