Since the formatter always depended on devShell packages, building the
formatter involved building all the devShell packages, which can be
slow.
In the case where formatters are set to plain strings, such as
"nixpkgs-fmt", depending on the devShell packages is necessary in order
to put the formatting utility on the path. But when a formatters option
is set to an package, such as "${pkgs.nixpkgs-fmt}/bin/nixpkgs-fmt",
then that formatter option does not depend on the devShell packages.
Flakelight now detects if any of the configured formatters are using the
first form, and only if so does it add the devShell packages dependency.
This allows the first form to still work, without incurring a cost for
flakes that only use the second form.
Users can use the second form for all formatters options if they wish to
not build the devShell packages when using the formatter.
`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.
The formatter uses `devShell.packages` for its path which is not
available when devShell is null. A default value of empty list should be
used when devShell is null.
