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/riscv64-linux/default.nix

@@ -0,0 +1,38 @@
+{
+  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 = {
+    nixosConfigurations = lib.attrsets.mergeAttrsList (map (it: it.nixosConfigurations or {}) dataWithoutPaths);
+    packages = lib.attrsets.mergeAttrsList (map (it: it.packages or {}) dataWithoutPaths);
+    # colmena contains some meta info, which need to be merged carefully.
+    colmena-meta = {
+      nodeNixpkgs = lib.attrsets.mergeAttrsList (map (it: it.colmena-meta.nodeNixpkgs or {}) dataWithoutPaths);
+      nodeSpecialArgs = lib.attrsets.mergeAttrsList (map (it: it.colmena-meta.nodeSpecialArgs or {}) dataWithoutPaths);
+    };
+    # colmena's per-machine data.
+    colmena = lib.attrsets.mergeAttrsList (map (it: it.colmena or {}) dataWithoutPaths);
+  };
+in
+  outputs
+  // {
+    inherit data; # for debugging purposes
+
+    # NixOS's unit tests.
+    # unit-tests = haumea.lib.loadEvalTests {
+    #   src = ./tests;
+    #   inputs = args;
+    # };
+  }