[Bug Report] the ElTable component cannot be extended. There will be some errors when referring to ElTable for extension

### Element UI version
2.15.3

### OS/Browsers version
windows10/chrome

### Vue version
2.6.14

### Reproduction Link
https://codepen.io/pen/Cannot be reproduced using codeopen

### Steps to reproduce
step 1:
import ElementUI from 'element-ui';
import 'element-ui/lib/theme-chalk/index.css';
Vue.use(ElementUI)

step 2:
import ElTable from "element-ui/packages/table/src/table";
export default {
name: 'ExtendTable',
extends: ElTable
}

step 3:
import ExtendTable from "./extend-table"
let Commons = {
install: function (Vue) {
Vue.component(ExtendTable.name, ExtendTable);
}
}
export default Commons;

### What is Expected?
It can use mixins or extensions to extend components normally

### What is actually happening?
When using mixins or extensions to extend components, an error will be reported,Cannot be reproduced using codeopen