Identifying unused resources in the manifest

[mattgarrish]

By adding a class of resources that can travel in an epub without having to be used directly in the rendering (e.g., data sets), it's not easy to determine if resources don't belong in the container anymore (e.g., if a publisher has updated their ebook and stopped using a resource).

It's not technically a violation of the specification to have resources in the manifest/container that aren't in use, but it could be helpful to identify these from a cleanup perspective. Adding to the complexity, however, is that it's not always possible to determine what resources are used by scripts. It could result in false positives.

To assist in identifying which resources a publisher intends to have travel in the container without being used, we could mitigate some of the difficulty by, for example, having an "exempt" property on the resources' manifest items. But this won't fully solve the problem.

Pull request #2507 was a rushed attempt to add such a property, but I'm closing it off as we don't have the time to fully consider this issue for the 3.3 revision. When we have more time, we might want to look at this issue again and see whether it has any benefit or if it only adds complexity.

Epubcheck could always emit usage messages about resources it thinks are unused so authors can review; it just couldn't stop emitting the message for subsequent runs without some means of telling it a resource is okay.