Issue #57: Cache metadata lookup tables
Rather than deriving the required lookup tables
from the repo metadata every time fedmod runs,
instead run the derivation code as part of the
fetch-metadata command, and cache the result as
JSON files.
There are still lots of opportunities for start-up
time optimisations here, this is mainly aimed
at separating fedmod's notion of a "working data set"
from the raw repo level metadata that DNF uses.