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://secure.phabricator.com/D16316