Check vimPlugins coverage in one nix eval before migrating

Before moving a vim.pack/lazy.nvim plugin list into Nix, check which plugins nixpkgs lacks with one eval: nix eval --impure --raw --expr 'let pkgs = import (builtins.getFlake "nixpkgs") {}; in builtins.concatStringsSep " " (builtins.filter (n: !(pkgs.vimPlugins ? ${n})) [ "snacks-nvim" "harpoon2" ... ])'. Empty output means all present. Attribute names differ from repo names (dots become dashes, harpoon v2 is harpoon2), so map names first. In one 41-plugin config every plugin existed, which drops a Nix migration from days to hours and makes nixCats unnecessary.