An external Package-arch-specific file makes life hard

The Soyuz Package-arch-specific file (which is used to override what architecture a package builds on no matter what the package says) is currently an external file on disk which is a checked-out Bazaar branch. This should not be checked out, but instead kept inside Launchpad's database. It should also be pre-parsed, since it is currently parsed for every single build that's created. Ideally, something should detect that branch changing and kick off a parsing job.