Fix dynamic Import issue with function-defined externals

[mununki]

Fixes #6985

This PR fixes that the dynamic imports don't work correctly with external declarations that define functions.

@module("mylib") external f: string => unit = "default"
let mylibF = Js.import(f)

compiled to

let mylibF = import("mylib").then(m => m.default);

[zth]

I'm not familiar with this part of the code base, but it looks good to me from what I can tell. @cristianoc ?