Motivation for this change

The filtering of the coqPackages attribute set visits too much attributes. For instance:

$ nix-build -A coqPackages.coq.ocamlPackages.ocaml
error: bin_prot-112.24.00 is not available for OCaml 4.06.1
(use '--show-trace' to show detailed location information)

There is no reason why the filtering should recursively visit coq.ocamlPackages. This patch ensures that the filtering only recursively traverses the sets that are marked with recurseIntoAttrs.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.