More consistent naming with Swift Protobuf

This update adds a dependency on SwiftProtobufPluginLibrary with the goal of using the SwiftProtobufNamer class to generate names that are consistent with the ones used by the Swift Protobuf project. Currently only message names are affected.

It also manually imports code used to determine file names for generated code to allow the default path names of files generated by protoc-gen-swiftgrpc to be more consistent with those from protoc-gen-swift.