modules-closure: don't ignore all deps if the first is builtin

[lheckemann]

Motivation for this change

The previous version resulted in modules being omitted entirely
because the first dependency listed for them by modprobe was
built-in. Now, only the actual built-in modules will be ignored when
building the closure.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

TODO: test if this fixes the issue for me in practice it does!

@ofborg test boot

[xaverdh]

#92081 for prior art

[lheckemann]

That's much better!