[swift_proto_library] Generated types are always public

[Tyler Vick]

Currently the Visibility generation option is hardcoded to Public for all protobuf generation:

rules_swift/swift/internal/swift_protoc_gen_aspect.bzl

Line 185 in 7ea1970

 protoc_args.add("--swift_opt=Visibility=Public") 

Certain use cases require a non-public access level for generated objects. For example, when creating a library without exposing messages to the consumer.

[Thomas Van Lenten]

This is because each swift_proto_library ends up being a module. If it was anything other than public how would it be used?