Build plugin fails when two .proto files have the same name

[glbrntt]

If two .proto files have the same name, e.g.:

• foo/bar.proto
• bar/bar.proto

Then the build plugin will generate files call foo/bar.grpc.swift and bar/bar.grpc.swift, when compiled these will both result in object files called bar.grpc.swift.o(the path prefix is dropped) which causes compilation to fail.

We currently pass FileNaming=FullPath to the protoc plugins, we should switch to PathToUnderscores instead.

[glbrntt]

This was resolved by #47