Slim down flake inputs using flake-parts partitions

[srid]

https://flake.parts/options/flake-parts-partitions.html

[srid]

@roberth

In case if this interests you (as an user report):

I tried using flake-parts partitions to prune out dev-related inputs from top-level flake.nix so that nix run doesn't have to pull in and evaluate the unnecessary stuff. For that purpose alone, partitions work well.

But then, I tried adding Emanote docs (example) which adds a packages.site (static site). I can't actually expose this package to top-level flake (dev partition exports only devShells and checks); nor is there a way to actually access it for the CLI. This sounds like a design trade-off per hercules-ci/flake-parts#258

So, I will close this PR, and go back to the subflake approach (where things are fully decoupled and hooked up via --override-input) viz.: https://github.com/srid/haskell-flake/blob/3ab2a076aba01d932644f6f21e8aa507d28bb36b/.envrc#L2