Allow behaviors to load classes without being the main model of a capsule

[Tofandel]

Makes the logic for finding behavioral models part of the Capsule helper and allow to load classes without being the main model of a capsule

Fixes an issue with some behaviors simply not working if they are not part of the main model of the capsule

For example for HasSlugs of the class Package\Capsules\MyCapsule\MySubCapsule this is the order it will try to load classes in

1. App\Twill\Models\Slugs\MySubCapsuleSlug
2. Package\Capsules\MyCapsule\MySubCapsuleSlug // Will not fail if the model is a secondary model of the capsule
3. Package\Capsules\MyCapsule\Slugs\MySubCapsuleSlug
4. Package\Capsules\MyCapsule\MyCapsuleSlug // Will fail if the model is a secondary model of a capsule

Fix the method getSlugClass which had the wrong return type and was missing parenthesis (so it was trying to find a property rather than calling a method)