[Swift] RFC: Switch Swift namespace from public enum to ordinary concat with _ (...

[Swift] RFC: Switch Swift namespace from public enum to ordinary concat with _ (#6045)

This PR attempts to switch namespace from public enum back to ordinary
concat with _ in Swift. This kept style similar with protobuf, but
different from other popular style guide in Swift.

This is needed because previously, when we do `public enum`, we don't
really know when to declare and when to extend (extension). With namespace
implementation in this PR, there is no such ambiguity.