Deriving from multiple distroseries fails with duplicate package errors

To re-create:
1. Make a new distro and series
2. On the initialization page (+initseries):
  * Select natty as a parent
  * Select lucid as a parent
3. Select "zope" packageset from natty
4. Select "mythbuntu" packageset from lucid
5. Click initialize

What happens:
The initalization job fails with errors like:
zope.traversing 3.7.1-4 in lucid (version older than the zope.traversing 3.13.2-0ubuntu1 in series-3-1 published in series-3-1))

This is because when initializing the first distroseries, the code in _copy_publishing_records passes packages from ALL of the packagesets to the packagecloner instead of just the packages in the packagesets for the source series.