is there a function to parse a flake lockfile into a set?

let sources = parseFlakeLock (builtins.readFile ./flake.lock);

something like this

Yes, in fact flake-compat is based on this principle. See examples here: https://github.com/nix-community/flake-compat

Basically: builtins.fromJSON (builtins.readFile ./flake.lock);

so I'm just doing this rn

  # fetch from flake.lock
  with (builtins.fromJSON (builtins.readFile ../flake.lock));
  fetchFromFlakeLock = args:
    fetchFromGitHub {
      inherit (args.locked) owner repo rev;
      hash = args.locked.narHash;
    };
  servant = fetchFromFlake (nodes.servant);