As discussed in #214, source_created and source_changed didn't really
have clear definitions. In truth, their names and separation betray
their origins as model receivers in earlier versions. The "source group"
abstraction helped us get away from thinking about things exclusively in
terms of models, but these remained as an artifact.
Differentiating between when the generated file content is required and
when the generated file is just required to exist gives us more
flexibility with strategies.
Changed my mind about 04aa72c1f9. It's
just a better description, even if different strategies can change the
behavior so it isn't really very cache-like.
This allows a sensible specialized interface for source groups, but also
for ImageKit to interact with specs using the generalized image
generator interface.
The registry module splits the work that specs.SpecRegistry
used to do into two classes: GeneratorRegistry and
SourceGroupRegistry. These two registries are wrapped in
Register and Unregister utilities for API convenience.
