Change to per-repo versioning

[devjgm]

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:

• Current repo version: v0.20.0
• Current GCS version: v1.11.x
• Current Bigtable version: v1.8.x

All libraries will be assumed to be Generally Available (GA) unless otherwise noted in the google/cloud/$library/README.md file.

[coryan]

I think until -common is merged into the repo we will need to manually sync and keep GCS and Bigtable version numbers. @devjgm if you agree, I can send an initial PR later today.

[devjgm]

I'm not sure I understand why we need to wait for -common to be merged into this repo. I was thinking that we could, say today, bump the GCS and Bigtable version numbers to say 1.5.0. Then in our May release of -cpp, we'd release the whole repo at version 1.5.0. That would put the whole repo, and Bigtable and GCS, at version 1.5.0.

Let me know what I'm misunderstanding.

[coryan]

I think you are right, except for a couple of weird places, let me send a PR.