So we have a spec of the layout of the store, independent of the
instance of the store within ~/.cabal/store. This will make new store
handling code cleaner by not entangling it with other global ~/.cabal
things.

We may also in future want to allow global and per-user stores, and we
certainly want the ability to specify a store location outside of
~/.cabal (though this could be done with the existing code too).