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?

首先，使用所示的具名导入和 mangle 正则表达式重现默认构建和 UMD 构建，然后在禁用 mangling 的情况下比较它们生成的 require 调用。完成的标准是：对依赖项 `_fn` export 的调用在两个构建中都保持有效，同时本地名称仍然可以进行 mangling。

由索引模型根据 Issue 内容生成。