This edge case was inconsistent between setting outputs directly and
auto-loading. Setting directly did not support args, so just merged
sets, thus resulting in a __functor flake output. When autoloading, a
set with a __functor attr was treated as a function and passed module
args.
To correct this inconsistency, outputs now always supports taking args,
and has the autoloading behavior.
Code that relied on previous behavior is easy to fix, as the value needs
to be converted to a function, which can then return the set with
__functor.
Using `perSystem` to implement per-system attributes ties all the
per-system attributes together; all the `perSystem` functions must run
to determine output attrs. By generating them separately, the generation
can be done lazily.
