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(`-`).

hosts/darwin-harmonica/home.nix

@@ -0,0 +1,17 @@
+_: {
+  programs.ssh = {
+    enable = true;
+    extraConfig = ''
+      Host github.com
+        Hostname github.com
+        # github is controlled by harmonica~
+        IdentityFile ~/.ssh/harmonica
+        # Specifies that ssh should only use the identity file explicitly configured above
+        # required to prevent sending default identity files first.
+        IdentitiesOnly yes
+    '';
+  };
+  modules.editors.emacs = {
+    enable = true;
+  };
+}