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(`-`).

vars/default.nix

@@ -0,0 +1,6 @@
+{lib}: {
+  username = "ryan";
+  userfullname = "Ryan Yin";
+  useremail = "xiaoyin_c@qq.com";
+  networking = import ./networking.nix {inherit lib;};
+}