A few plugins need more complex dependency calculations than ikiwiki can do
on its own:

* Use of a version plugin should only make the page rebuild when it's built
  with a new version of ikiwiki.
* Some plugin might want to _always_ rebuild the page that uses it.
* If backlinks were turned into a plugin, it would need to make a page
  rebuild when its backlinks changed.

These suggest there should be a way for plugins to have hooks that tweak
the list of pages to rebuild.

Which in turn suggests that there should *be* a list of pages to rebuild;
currently there's not, and the best such an interface could do would be to
rebuild the pages even if they were already going to be rebuilt for some
other reason. (See [[optimisations]].)

It also suggests that plugins will want to examine pages and/or
[[store_data|plugin_data_storage]] about them to use in the dependency
calculations. For example, the version plugin would need to store info
about what pages use it.

> I [[fixed|done]] this without realizing it when I added the needsbuild hook!
> --[[Joey]]
