lib/modules: pass inputs.nixpkgs into evalNixvim

Expose our locked nixpkgs as the `nixpkgs.source` module options.

This only happens when `evalNixvim` is part of a lib that was provided
`flake` as an argument.

Stubbed the `nixpkgs.source` option for now. Eventually, this will be
used to construct `pkgs` internally. For now, it's purely informational.

lib/tests.nix

@@ -1,4 +1,5 @@
 {
+  self,
   pkgs,
   lib ? pkgs.lib,
   ...
@@ -47,6 +48,7 @@ let
       # NOTE: we are importing this just for evalNixvim
       helpers = import ../lib {
         inherit lib;
+        flake = self;
         # TODO: deprecate helpers.enableExceptInTests,
         # add a context option e.g. `config.isTest`?
         _nixvimTests = true;