`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.
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.
In addition to a regular bundler of the form `x: x`, this allows setting
bundler options to a function of the form `pkgs: x: x` which is passed
the package set for the system (for example: `{ hello, ... }: x: hello`
to always return hello). This allows, in particular, an autoloaded
bundler in its own file to access the package set.
This is non-trivial as we must tell `x: x` and `pkgs: x: x` apart.
Fortunately, given some derivation `drv`, `pkgs // drv` is a valid
derivation and a set with attr names matching pkgs. When applying this
to the function, if it returns a derivation, it was of the `x: x` form,
and if it returns a function, it was of the `pkgs: x: x` form.
In order to prevent IFD when evaluating the flake if the bundler is of
form `x: x` and uses IFD, we determine the form and apply pkgs when
applying the bundler instead of during flake evaluation. This is done by
wrapping the bundler.
Of note, we cannot rely on `builtins.functionArgs`, since
`pkgs: { hello, ... }: (x: hello) pkgs` is the same as the inner
function but with args hidden.
