Change to per-repo versioning
Overview
As described in #3612, this repo is changing to a monorepo that will contain the code for all the Cloud C++ client libraries. Each library will live in a per-library directory, like google/cloud/$library. The repo itself will have a version number that corresponds with the versions on the https://github.com/googleapis/google-cloud-cpp/releases page. But each individual library will not contain their own version numbers. To be clear (and visually skimmable):
There will be a single per-repo version number, NOT per-library version numbers.
This differs from today where there is a repo version number, and each individual library also has their own version number. For example, today we have:
All libraries will be assumed to be Generally Available (GA) unless otherwise noted in the google/cloud/$library/README.md file.