We have an idiom in many tests where we import the "default export" from
a module and then access the `$imports` property on that
function/class/object in order to mock or unmock imports. For example:

```
const Widget = require('../widget');

beforeEach(() => {
  Widget.$imports.$mock(...);
});

afterEach(() => {
  Widget.$imports.$restore();
});
```

This won't work when the module under test is converted to an ES module
because the `$imports` object will become a separate export from the
module rather than a property of the default export.