Cleanup directory structure 

• Clean up `GoogleDataTransport/` to align with [guidelines](https://github.com/firebase/firebase-ios-sdk/blob/master/AddNewPod.md#directory-structure)
• [Package.swift TODO](https://github.com/google/GoogleDataTransport/blob/a7b0ff508e716307116df47b652c7704141d37fc/Package.swift#L41)
• [Conversation here](https://github.com/google/GoogleDataTransport/pull/20#discussion_r614181690)