Support generating multiple `flake.nix` files

[folliehiyuki]

My use case is for a project using partition feature of flake-parts. It's basically a flake repo containing a bunch of child flake directories (that define only inputs). It'd nice to be able to mass generate flake.nix files for all the flakes within a repo from a single flake-file config.

A WIP concept might look like this:

{
  # Define inputs for the main flake, generating `<intoPath>/flake.nix`
  flake-file.inputs = {...};

  # Generate `<intoPath>/dirA/dirB/flake.nix` file
  flake-file."dirA/dirB".inputs = {...};
}