Summary
Fix generation for OneOf unions involving alias user types, especially when struct:pkg:path places the member in a different package.

Rationale
In Go, an interface that has an unexported method can only be implemented by types in the same package. Previously the union marker method could end up associated with a member type that lives in a different package than the union interface, which makes it impossible for that member to satisfy the interface. In addition, when multiple union branches share the same underlying type, generated type switches can produce duplicate cases.

This change generates a small, per-branch wrapper type in the package where the union interface lives (or a common shared package for shared user-type unions) and attaches the marker method to that wrapper. The wrapper is a distinct type defined locally, so it can satisfy the interface without cross-package unexported methods, and it gives each branch a unique concrete type for type switches. When a union member already lives in the same package and is unique, the marker is attached directly to the member to preserve existing pointer vs value receiver semantics.

Changes

• Generate wrapper types for union members when they are cross-package, duplicated, or primitives.
• Attach union marker methods to wrappers; otherwise attach directly to the member.
• Preserve pointer/value receiver semantics for existing user types.
• Update gRPC transforms to reference wrappers in both directions where required.
• Avoid extra whitespace in marker templates to keep golden files stable.

Compatibility
No DSL changes. Only generated code changes; behavior remains the same for unions that did not require wrappers.

Testing
All existing Goa tests pass locally.