bug: import external libs does not add 'js' extension for esm

[Lonli-Lokli]

Is there an existing issue for this?

• I have searched the existing issues

Description of the bug

So I've tried to add this to my fork with tsup and external imports are not processed

Steps To Reproduce

1. clone https://github.com/Lonli-Lokli/react-mosaic/tree/esm
2. add plugin
3. run 'yarn build'

Expected behavior

import countBy from "lodash-es/countBy";
transformed into
import countBy from "lodash-es/countBy.js";

Screenshots

No response

Additional context

No response

[favna]

This is probably a duplicate of #113 but I haven't taken time to investigate it yet. I find it odd though because I really thought this was fixed already by the package matching. What happens if you try to write it as import { countBy } from 'lodash-es';?