Migrate DistributionSourcePackage database class to be backed by the database

DistributionSourcePackage already exists in the database, but it isn't
used by the database class in the LP tree. DSP is a "virtual" thing.

However, once the fix for bug #43893 and/or bug #273495 (development
started under the banner of bug #43893 and the other bug was filed
later to cover those specific changes), a new database class will
exist - DistributionSourcePackageInDatabase - that does use the DSP
table.

This has been done so that we can select and update the
bug_reporting_guidelines column. This database class is not accessible
outside of the module in which it's defined, and access to the
bug_reporting_guidelines column is managed entirely via a property on
the non-database-backed DistributionSourcePackage class.

Phew. In short, it means that the interface to DSP has not changed.

The justification is that there wasn't time to do the complete job of
migrating DSP to being fully backed by the database. As you'll see, I
don't think this short-term measure will complicate the migration from
a virtual DSP class to a database-backed one greatly.

A plan was discussed during the pre-implementation phase of bug
#43893:

 * Need to create new `DistributionSourcePackage` rows in the Soyuz
   upload code at the same time as it creates new `SourcePackageName`
   records. In addition to a bulk fix at roll-out time.

 * Create a new `IDistributionSourcePackageSet` utility to find and
   create these things.

 * Update the few places that create `DistributionSourcePackage`
   objects on the fly to use the utility.

 * It'll be important to check if there are any nasty interactions
   w.r.t. `DistributionSourcePackageCache`. This is updated daily in a
   cron job, `cronscripts/update-pkgcache.py`.

 * Ideally we can ignore this cache for now, and everything will
   work. However, it probably makes sense to fold everything in
   `DistributionSourcePackageCache` into `DistributionSourcePackage`.

BjornT commented on this at the time:

  I'm not convinced we need a IDistributionSourcePackageSet
  utility. We should get them through IDistribution, and there should
  be one API to create both the DistributionSourcePackage and
  SourcePackageName at the same time.