`nullOr`'s merge function requires definitions to all be null or all be
non-null. It was being used where the intent was that null be used as a
value representing unset, and as such the merge should return null if
all definitions are null and ignore nulls otherwise. This adds a type
with that merge semantics.
The set of args passed to modules is useful for more than just
autoloading. This renames it appropriately and makes it available
through pkgs arguments as well.
