Exported PropTypes shapes get lost in translation

Hi there,
Thank you for the great work you've done with microbundle. For the record, here is how I'm using the lib:
`microbundle --jsx React.createElement --no-compress --format modern,cjs`

I'm using react in version 17 and microbundle in version 0.12.0-next.9

I've got a PropType shape that is used in several definitions:

```js
import PropTypes from "prop-types";

const LocaleShape = PropTypes.shape({
// two-char language code ("en", "fr", "de")
tag: PropTypes.string.isRequired,
// human readable language description
language: PropTypes.oneOf([PropTypes.object, PropTypes.string]).isRequired,
//latin, cyrillic, etc.
script: PropTypes.string,
// two-char region code ("US", "CA", "GB", "CH")
region: PropTypes.string,
// text direction / orientation: right to left, left to right
orientation: PropTypes.oneOf(["rtl", "ltr"]),
// optional emoji string for displaying a flag 🇨🇬 🇲🇼 🇹🇩
flag: PropTypes.string,
// String that can be used to search for a locale
keywords: PropTypes.string,
// locale identifier of the fallback locale
fallback: PropTypes.string,
// optional identifier. If not specified, locale identifier will result in the concatenation
// of tag and region, with a dash separator: "en-GB", "fr-CA", "nl-BE"
id: PropTypes.string,
});
export default LocaleShape;
```

This shape is imported in different files:
```js
LocaleChooser.propTypes = {
value: PropTypes.oneOfType([LocaleShape, PropTypes.arrayOf(LocaleShape)]),
locales: PropTypes.arrayOf(LocaleShape),
}
```

When the bundle is built, it looks like the exported PropType shape is turned into `null`, as explain the warnings I receive in console:

```
Warning: Failed prop type: Invalid prop `locales[0].language` of value `[object Object]` supplied to `LocaleChooser`, expected one of [null,null].
```

This warning is suppressed once I remove the imports and duplicate the LocaleShape in every file that requires it. Am I missing a parameter in the command?

Thanks for your help