merging and spliting projects needs better tag managment

[Curtis Hovey]

Fixing the remaining limitations with tags, or adding all the required
tools to support splitting and merging products needs to be scheduled
so we can support larger projects like Launchpad (Hmm... in this case
[creating launchpad-registry from a subset of launchpad-foundations],
splitting wouldn't have been possible without tags, so I guess that
should be the priority even if we want to promote the multiple-product
model for administrating large codebases like Launchpad).

Another interesting point doing this - making changes based on tags
can technically be dangerous, as bug tags are on the bug and thus
shared between bugtasks. If two projects use the same bug tag (eg. I
suspect plenty of projects will want a 'ui' tag like Launchpad uses),
then you could end up changing bugtasks you didn't want to. I don't
know if this edge case will ever bite anyone in reality though, and I
doubt we want to move tags to the BugTask just in case as this would
adversely affect usability.