home: create 'modules/home' folder

Consolidating all modules under the same path, to clear out the
top-level directory.

modules/home/mail/default.nix

@@ -0,0 +1,31 @@
+{ config, lib, ... }:
+let
+  cfg = config.my.home.mail;
+
+  mkRelatedOption = desc: lib.mkEnableOption desc // { default = cfg.enable; };
+in
+{
+  imports = [
+    ./accounts
+    ./himalaya
+    ./msmtp
+  ];
+
+  options.my.home.mail = with lib; {
+    enable = my.mkDisableOption "email configuration";
+
+    himalaya = {
+      enable = mkEnableOption "himalaya configuration";
+    };
+
+    msmtp = {
+      enable = mkRelatedOption "msmtp configuration";
+    };
+  };
+
+  config = {
+    accounts.email = {
+      maildirBasePath = "mail";
+    };
+  };
+}