Caches information about resources on the classpath. In addition, acts as an invalidation hub for any resources for
which information is obtained (when any of the resources are changed, invalidation listeners are notified so they can
clear their caches).
Note that the name and role of this class changed (and diminished) quite a bit in Tapestry 5.3. It is now focused on
determining which files require a digest, and which what the digests are for resources.
Further, in 5.4, this service has been stripped down to an unused placeholder; it only present to prevent
errors where modules have contributed a String extension to protected (even though the service is internal). As of
5.4, all assets have a built-in checksum in the URL directly (in 5.3 and earlier, the URL included the application