Finalize the versions of the unstable modules in V2

Two main questions:

1. Should they be versioned independently of each other, or should they all be released in a single version together, like V1 modules are today?
2. What should be the exact version - `2.0.0-<prerelease-tag>`, or `0.x.y`?

[This issue](https://github.com/aws/aws-cdk/issues/15582) might also weigh in on that decision.