Fix code generation for Name trait

[tinrab]

Fixes a bug in #931.

In code generator, package field was used to track nesting of messages and didn't represented the protobuf's "package" name. Nested names are now contained in type_path and package refers to file's package.

[BKDaugherty]

Closes #946 .

(Edit: Oh lol, maybe I don't have permissions for that 🙃 )

Thanks @tinrab for both the quick fix and adding the feature to prost-build in the first place!

[LucioFranco]

Thanks!