refactor: Use haumea for filesystem-based module system for flake outputs

refactor: Use hyphen(`-`) for variable names & folder names(except Python), replace all unserscore(`_`) with hyphen(`-`).

outputs/aarch64-darwin/default.nix

@@ -0,0 +1,30 @@
+{
+  lib,
+  inputs,
+  ...
+} @ args: let
+  inherit (inputs) haumea;
+
+  # Contains all the flake outputs of this system architecture.
+  data = haumea.lib.load {
+    src = ./src;
+    inputs = args;
+  };
+  # nix file names is redundant, so we remove it.
+  dataWithoutPaths = builtins.attrValues data;
+
+  # Merge all the machine's data into a single attribute set.
+  outputs = {
+    darwinConfigurations = lib.attrsets.mergeAttrsList (map (it: it.darwinConfigurations or {}) dataWithoutPaths);
+  };
+in
+  outputs
+  // {
+    inherit data; # for debugging purposes
+
+    # NixOS's unit tests.
+    # unit-tests = haumea.lib.loadEvalTests {
+    #   src = ./tests;
+    #   inputs = args;
+    # };
+  }

outputs/aarch64-darwin/src/fern.nix

@@ -0,0 +1,35 @@
+{
+  # NOTE: the args not used in this file CAN NOT be removed!
+  # because haumea pass argument lazily,
+  # and these arguments are used in the functions like `mylib.nixosSystem`, `mylib.colmenaSystem`, etc.
+  inputs,
+  lib,
+  mylib,
+  myvars,
+  system,
+  genSpecialArgs,
+  ...
+} @ args: let
+  name = "fern";
+
+  modules = {
+    darwin-modules =
+      (map mylib.relativeToRoot [
+        # common
+        "secrets/darwin.nix"
+        "modules/darwin"
+        # host specific
+        "hosts/darwin-${name}"
+      ])
+      ++ [];
+    home-module.imports = map mylib.relativeToRoot [
+      "hosts/darwin-${name}/home.nix"
+      "home/darwin"
+    ];
+  };
+
+  systemArgs = modules // args;
+in {
+  # macOS's configuration
+  darwinConfigurations.${name} = mylib.macosSystem systemArgs;
+}