Add PackagesPackage

Summary:
Ref T8116. A package has:

• a publisher (like "Phacility"), from the previous revision;
• a name (like "Arcanist");
• a package key (like "arcanist").

The package key is immutable, like the publisher key.

This gives a package a full key like "phacility/arcanist".

Policy stuff:

• You must be able to view a publisher to view a package (currently, everyone can always see all publishers).
• You must be able to edit a publisher to create a new package inside it.
• Packages have separate view/edit permissions.

This still does nothing interesting.

Test Plan:

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T8116

Differential Revision: https://secure.phabricator.com/D16315