esModuleInterlop not used in secondary file

[EndBug]

My code has two files, that look like this:

// main.ts
import path from 'path' // This uses esModuleInterlop, and doesn't cause issues
import { setOutput } from './util'

path.normalize('') // OK
setOutput('a', 'b') // Cannot read property 'setOutput' of undefined

// util.ts
import core from '@actions/core' // This should use esModuleInterlop too, but I get an error

export function setOutput(a, b) {
  return core.setOutput(a, b)
}

The error I'm showing in the code ("Cannot read property 'setOuput' of undefined") only happens when I use the build, there are no compile errors. Since this is the code of a GitHub Action, I have no way of running this code locally before NCC compiles it.

I know that the issue is with esModuleInterlop because I solved it by using import * as core from '@actions/core'

Do you have any clue on what could I have been doing wrong?

[guybedford]

This seems like a duplicate of #626, will address this there.