R1:ee37eca2e4c2

Add PackagesVersion

Summary:
Ref T8116. A version has:

- a package (like "Arcanist") which it belongs to;
- a name (like "v3.1.5").

The name is immutable and unique, like the package key and publisher key.

Policy stuff:

- Versions have the exact same policies as their packages.
- You must be able to edit a package to create new versions of it.

This is still entirely uninteresting.

Test Plan: {F1731703}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8116

Differential Revision: https…