Feature request: Flag to Mark Generated Code as Package Private

[bpholt]

Motivation: allowing libraries to generate smithy4s clients intended for use only within the library, avoiding binary compatibility issues caused by differences in the generated code over time.

Such a feature would remove the need for the Scalafix referenced in #1589.

[Baccata]

I'm not necessarily opposed to it but you'd have to propose semantics (in this issue) and implement them yourself once @kubukoz and I have agreed to it.