treewide: avoid passing pkgs to our lib

In the two places where our "helpers" lib is exclusively internal
(flake module args and building the docs), we no longer supply `pkgs`.

In the other 4 locations, we now note why we still do in a comment.

wrappers/_shared.nix

@@ -46,6 +46,7 @@ in
   config = mkMerge [
     {
       # Make our lib available to the host modules
+      # NOTE: user-facing so we must include the legacy `pkgs` argument
       lib.nixvim = lib.mkDefault (import ../lib { inherit pkgs lib; });
 
       # Make nixvim's "extended" lib available to the host's module args

wrappers/standalone.nix

@@ -7,6 +7,7 @@ default_pkgs: self:
   module,
 }:
 let
+  # NOTE: user-facing so we must include the legacy `pkgs` argument
   helpers = import ../lib { inherit pkgs lib _nixvimTests; };
 
   inherit (helpers.modules) evalNixvim;