`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.
Many attributes can take moduleArgs when auto-loaded in order to
facilitate access to them from other files. Those same attributes could
not take moduleArgs when included directly, which was inconsistent.
With this change, all attributes that could take moduleArgs when
auto-loaded can now always do so. Auto-loading no longer needs special
cases.
