neovim-require-check-hook: remove $out from rtp

Previously, the tested plugin appeared in `rtp` under two paths:
- `/nix/store/<hash>-vimPlugins-my-plugin` from `$out`
- `/build/.local/share/nvim/site/pack/nvimRequireCheckHook/opt/testPlugin`
  from `packadd testPlugin`

This was causing problems for plugins that use
`vim.api.nvim_get_runtime_file()`. For example, plugins that use
`blink.lib` to load rust modules fail the require check because
`blink.lib` uses this function to locate rust shared libraries and
raises an error when duplicate libraries are found.

The plugin is already loaded via `packadd testPlugin`, so adding `$out`
to `rtp` is unnecessary. Remove `$out` from `rtp` to avoid duplicate
runtime entries, and add a regression test to ensure the plugin appears
in `rtp` only once.

Co-authored-by: Austin Horstman <khaneliman12@gmail.com>

pkgs/applications/editors/neovim/tests/default.nix

@@ -558,4 +558,25 @@ pkgs.lib.recurseIntoAttrs rec {
       EOF
     '';
   };
+
+  nvim_require_check_rtp_no_duplicate = vimUtils.buildVimPlugin {
+    pname = "neovim-require-check-rtp-no-duplicate-test";
+    version = "0";
+    src = runCommandLocal "neovim-require-check-rtp-no-duplicate-src" { } ''
+      mkdir -p "$out/lua/require-check-rtp-dedup"
+      cat > "$out/lua/require-check-rtp-dedup/init.lua" <<'EOF'
+      local target = "lua/require-check-rtp-dedup/init.lua"
+      local matches = vim.api.nvim_get_runtime_file(target, true)
+      if #matches ~= 1 then
+        error(
+          ("expected plugin on runtimepath exactly once, found %d:\n%s"):format(
+            #matches,
+            table.concat(matches, "\n")
+          )
+        )
+      end
+      return {}
+      EOF
+    '';
+  };
 }

pkgs/applications/editors/vim/plugins/hooks/neovim-require-check-hook.sh

@@ -98,9 +98,9 @@ run_require_checks() {
         if [ "$skip" = false ]; then
             echo "Attempting to require module: $name"
             if @nvimBinary@ -es --headless -n -u NONE -i NONE --clean -V1 \
-                --cmd "set rtp+=$out,${deps// /,}" \
-                --cmd "set rtp+=$out,${nativeCheckInputs// /,}" \
-                --cmd "set rtp+=$out,${checkInputs// /,}" \
+                --cmd "set rtp+=${deps// /,}" \
+                --cmd "set rtp+=${nativeCheckInputs// /,}" \
+                --cmd "set rtp+=${checkInputs// /,}" \
                 "${luaPathArgs[@]}" \
                 --cmd "set packpath^=$packPathDir" \
                 --cmd "packadd testPlugin" \