R1:704afea281b1

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.