standard for package and service names, printed and used

[iansmith]

Create a type for package+service names and a standard way of printing them, maybe (file.File)

Code that is handling these needs to be using a specific type internally. Calls like Locate() and Require() that pass around that information should do so with a protobuf defined type.

It's ok if the programming language binding uses two strings for now, it's more important to update the internal usages so that it can be changed by somebody who wants to use a different definition.