nix flake check: Allow modules attribute

This is intended as a module system "library" in the format

<flake>.modules.<class>.<name>

where class is e.g. "nixos" or "homeManager", and the name
is of the author's choice.

Modules that can be loaded in any module system application should
use the name "generic".

- Implemented in the module system in https://github.com/NixOS/nixpkgs/pull/197547

- Class parameter for checking: https://nixos.org/manual/nixpkgs/stable/index.html#module-system-lib-evalModules-param-class

src/nix/flake.cc

@@ -773,6 +773,7 @@ struct CmdFlakeCheck : FlakeCommand
                             || name == "homeConfigurations"
                             || name == "homeModule"
                             || name == "homeModules"
+                            || name == "modules"
                             || name == "nixopsConfigurations"
                             )
                             // Known but unchecked community attribute