Broken build due to mangled dependency props

I have some code that imports a named export of a dependency as follows:
```js
import { _fn } from '@x/y';
...
_fn(...); // calling _fn
```

With mangle configured with `"regex": "^_[^_]"`, the default build (`index.js`) transpiles this to:
```js
const z = require('@x/y');
...
z.a(...); // calling _fn???
```

Disabling mangling works as expected:
```js
const z = require('@x/y');
...
z._fn(...); // calling _fn
```

There is a similar problem with the umd build.

Any way around this?