support "closed" package label for closed plugins #391
Description
Once we have a labeling service in place, we're going to want to tackle the issue closed plugins (and eventually do something about themes too). Right now, a closed plugin doesn't come with enough data from .org to support a proper Plugin entry, and we don't keep the old Plugin record around, it just gets memory-holed forever. This is a major flaw in AC's architecture that needs to be addressed separately, but in the meantime we should consider having ClosedPlugin retain the old metadata if present. From that we can create Packages that won't show up in normal browses, but can still be queried directly, including attached labels. Each of those packages would then also be associated with a "closed" label.
Once AC starts keeping historical plugin metadata and not dropping it, then we can dispense with ClosedPlugin entirely, and just have hidden Packages that reference historical rows, with all the data currently on ClosedPlugin being label data for that package.