The Resources now front the file sets made available by the bundlers
file set factories, rather than letting the BundlableNode do this. This
is more natural from an OO point of view, and also has the side-effect
that the file-sets are now automatically cached for each resource,
meaning we don't need a deliberate caching mechanism.

In addition to this, the ShallowResources class was implemented
(DeepResources still to come) and a couple of bugs in BundleSetBuilder
were fixed along the way.