Fails when a depedency (module) requires a webpack "external" module

Awesome library! I've just found an edge case when workerizing a module that depends on a webpack external.

In my webpack config i have:
```javascript
module.exports = {
// ...
externals: {
'config': JSON.stringify(config),
},
// ...
}
```

and in my worker module I have:
```typescript
import * as config from 'config';
```

I'm using typescript in case that matters.

Here's the compilation error from webpack:
```bash
Module not found: Error: Can't resolve 'config' in '/directory/src'
@ ./src/MyWorker.ts 1:0-33
```