treewide: create global user list

lib/default.nix

@@ -0,0 +1,21 @@
+rec {
+  users = import ./users.nix;
+
+  adminSSHKeys = builtins.concatLists (
+    map (user: getSSHKeys user.name) (
+      builtins.filter (user: user.value.options.admin or false) (attrsToList users)
+    )
+  );
+
+  attrsToList = mapAttrsToList nameValuePair;
+  mapAttrsToList = f: attrs: builtins.attrValues (builtins.mapAttrs f attrs);
+  nameValuePair = name: value: { inherit name value; };
+  toList = x: if builtins.isList x then x else [ x ];
+
+  getSSHKeys =
+    username:
+    if (builtins.hasAttr "ssh" users.${username}) then
+      toList users.${username}.ssh
+    else
+      builtins.warn "user ${username} declared without ssh key" [ ];
+}