Allow to specify some flake outputs from Nickel

[YorikSar]

Add flake field to project.ncl that allows to specify certain flake outputs. For example, you can add lines like these to run hello as an app or a check:

flake.apps.hello.program = nix-s%"%{import_nix "nixpkgs#hello"}/bin/hello"%,
flake.checks.hello = import_nix "nixpkgs#hello",

I will be gragually migrating apps and checks from our flake.nix to project.ncl using this.

Also update flake inputs for the example to use newer Nickel required by the new changes.

Part of #58.

[thufschmitt]

I'm not sure I get it. Why are some outputs at toplevel and some under flake? Can't we just have everything at toplevel in the flake? Or everything under flake if we want to hide it and add our own abstraction on top?

[YorikSar]

@thufschmitt I’m not sure we want to expose everything at top level, so I’ve decided to namespace it. Mixing flake API with Organist one doesn’t feel right to me because it’s less declarative, less abstract and less typed. I feel like flake section should be an “output” of Nickel part.
If you want, I can move it to the top level now, or we could do that at any point later if we decide that’s better.

[thufschmitt]

Fair enough. Let's keep our distances from the Nix interface for the time being.

We are somewhat tied to it though since Nix is our entry point. Not sure how to best juggle this in the long run.