vars: add .exists to files

nixosModules/clanCore/vars/interface.nix

@@ -304,6 +304,15 @@ in
                       description = "The unix file mode of the file. Must be a 4-digit octal number.";
                       default = "0400";
                     };
+                    exists = mkOption {
+                      description = ''
+                        Returns true if the file exists, This is used to guard against reading not set value in evaluation.
+                        This currently only works for non secret files.
+                      '';
+                      type = bool;
+                      default = if file.config.secret then throw "Cannot determine existance of secret file" else false;
+                      defaultText = "Throws error because the existance of a secret file cannot be determined";
+                    };
                     value =
                       mkOption {
                         description = ''

nixosModules/clanCore/vars/public/in_repo.nix

@@ -25,7 +25,7 @@ in
       );
       value = mkIf (file.config.secret == false) (
         # dynamically adjust priority to allow overriding with mkDefault in case the file is not found
-        if (pathExists file.config.flakePath) then
+        if file.config.exists then
           # if the file is found it should have normal priority
           readFile file.config.flakePath
         else
@@ -34,6 +34,7 @@ in
             throw "Please run `clan vars generate ${config.clan.core.settings.machine.name}` as file was not found: ${file.config.path}"
           )
       );
+      exists = mkIf (file.config.secret == false) (pathExists file.config.flakePath);
     };
   };
 }