The existing implementation of the interface (backed by the file
system) has moved into cmd/lumi. The deployment service will start to
provide its own version.
This refactors the engine so all of the APIs on it are instance
methods on the type instead of raw methods that float around and use
data from a global engine.
A mechcanical change as we remove the global `E` and then make
anything that interacted with that in pkg/engine to be an instance
method and the dealing with the fallout.
