neovimUtils: improve error message

Before:

    error: The option `plugins."[definition 1-entry 1]".toto' does not exist. Definition values:
       - In `<unknown-file>': "tata"

After:
    error: The option `plugins."[definition 1-entry 1]".toto' does not exist. Definition values:
    - In `pkgs/applications/editors/neovim/plugin-submodule.nix': "tata"

I've renamed "module.nix" to "plugin-submodule.nix" in case we want to
add a similar module for the top-level wrapper.

pkgs/applications/editors/neovim/utils.nix

@@ -82,7 +82,7 @@ let
       neovimConfig =
         structuredConfigure:
         let
-          module = import ./module.nix;
+          module = import ./plugin-submodule.nix;
           # Generate init.vim configuration
           cfg = (
             lib.evalModules {
@@ -97,6 +97,7 @@ let
 
       checked_cfg = neovimConfig {
         inherit plugins;
+        _file = "pkgs/applications/editors/neovim/plugin-submodule.nix";
       };
 
       pluginsNormalized = checked_cfg.plugins;