Scala 3: don't make the self ref vals private within

[KacperFKorban]

When creating symbols, their privateWithin has to be a type.
When defining zio-schema derivation e.g.

given x: Schema[ArangoError] = DeriveSchema.gen[ArangoError]

Symbol.spliceOwner of the derivation macro was x. Which is not a correct value for privateWithin.

This property will now be enforced with scala/scala3#17352

EDIT: Also after this fix -Xprint should work for programs using zio-schema derivations.

[jdegoes]

Thank you! Can you run sbt docs/generateReadme to update the README, then the CI build can make further progress!

[KacperFKorban]

@jdegoes rebased, the CI is green now

[jdegoes]

@KacperFKorban Thank you for your pull request and detailed explanation of the issue!