Instead of having to manually stub packages that should not be
downloaded we instead automatically stub all packages (except a small
list of whitelisted ones). Tests can re-introduce the real package by
using the `realPkgs` module argument.
This adds two new options: 'programs.neovim.coc.{enable,settings}`.
These settings offer a simple interface over `xdg.configFile."nvim/coc-settings.json`,
using the standard Nix' syntax instead of a multiline string.
