Add support for protobuf `package` specifier

[deepfriedret]

Hi, Protobuf has this package specifier which works like C# namespaces. In fact, C# generator use this to define namespace if csharp_namespace is not presented. It will be very helpful if the derived Protobuf schemas come with specifier. Any plan on implementing this? Thanks!

[Xpl0itR]

https://github.com/Xpl0itR/protodec#limitations

Package names are not preserved in protobuf compilation so naturally we cannot recover them during decompilation

[deepfriedret]

I understand that it may not be preserved in the present of csharp_namespace, but we can just use the namespace as package names? I think having that as an option will be nice.