Default to on_demand syncing
Since syncing all of PyPI would result in at least 6.1 terabytes of disk space being used , it is probably best that we default to on-demand download policy.
However, we should document this very clearly. If PyPI goes offline or specific packages become unavailable then packages which have not yet been downloaded won't be installable.
We should also give the user a fair warning in the documentation about what to expect if they try to perform an immediate-mode sync.