[bug report] introduces some components, but the final result of webpack packaging is that the entire library is introduced, and tree shaping is invalid

### Element UI version
2.13.2

### OS/Browsers version
macOS

### Vue version
3.0.0-beta.19

### Reproduction Link
https://jsfiddle.net/mmx38qxw/

### Steps to reproduce
引入某个组件，直接看打包的bundle即可

### What is Expected?
package.json 中应该添加module字段，导出es模块

### What is actually happening?
"main": "lib/element-ui.common.js"。这个配置根本就没办法做tree shaking吧，这导致了引入部分组件，但是项目打包后体积很大。