Expose qcal's JSON configuration through programs.qcal.settings and
per-calendar entries through accounts.calendar.accounts.<name>.qcal.settings.
Generated Calendars entries and the account remote projections use
mkDefault so users can override or extend them. timezone and
defaultNumDays become renamed aliases of the settings keys.
Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
