js.Build's externals option does not work for /assets

[rnwst]

Description and steps to reproduce

Under assets, I have a JS file script.js with contents

import '/external.js';

The file external.js has contents

console.log('This file should not be bundled!');

In my template, the script is included as follows:

{{- /* Make sure the resource "external.js" is published. */}}
{{- (resources.Get "external.js").Publish }}

{{- $script := resources.Get "script.js" }}
{{- $opts := dict "format" "esm"
                  "externals" (slice "/external.js")
}}
{{- $script = $script | js.Build $opts }}
  <script type="module" src="{{ $script.Permalink }}"></script>

Actual behaviour

The externals option seems to have no effect, and the file external.js is included in the bundled script.js. The bundled file looks as follows:

// ns-hugo-imp:/home/rnwst/src/hugo-test/assets/external.js
console.log("This file should not be bundled!");

Expected behaviour

external.js should not be bundled, and the import statement in script.js should be left untouched.

What version of Hugo are you using (hugo version)?

$ hugo version
hugo v0.140.0+extended linux/amd64 BuildDate=unknown

[rnwst]

I created a reproducing example here: https://github.com/rnwst/hugo-testing/tree/hugo-github-issue-13183

[bep]

I think I understand what's happening, and I suspect that this works fine for JS packages living in node_modules.

[rnwst]

Amazing, thank you for fixing this so quickly! Everything is working as expected on latest master.

Hugo is an awesome project, thank you very much for all your hard work!