Force users to explicitly enable auto typeclass derivation

[RustedBones]

I'm wondering if we can use the artifact change release to put some breaking changes into magnolify.

I noticed that depending on the module, the automatic derivation is not always in the same package:

• eg. scalacheck is in magnolify.scalackeck.auto._
• eg. avro is in magnolify.avro._

When looking at diffx (which is one common library using magnolia), they really separate the auto derivation:

Auto derivation might have a huge impact on compilation times, because of that it is recommended to use semi-auto derivation.

We should probably do the same. This would however break compilation of most projects using magnolify but can be resolved by adding the new auto import