Introduce a "prune" package in which we maintain a list of prune
functions that are registered. Known prune "content-types" are
included in a pre-defined order, after which additional content
can be registered.
Using this approach no longer requires the "RunPrune" functions
to be exported, and allows additional content-types to be
introduced without having to import those packages into the
system package, so keeping things more decoupled.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
02d578b637