[Bug Report] 【2.14.0版本】的【Popover弹出框】的 【自定义指令v-popover指向 Popover 的索引ref】有问题

### Element UI version
2.14.0

### OS/Browsers version
win7

### Vue version
2.5.20

### Reproduction Link
https://github.com/ElemeFE/element/issues/20381

### Steps to reproduce

export default {
data() {
return {
visible: false,
}
},
}

the same code in element-ui 2.14.0 is error【[Vue warn]: Failed to resolve directive: popover】
the same code in element-ui 2.11.1 is normal

### What is Expected?
期待解决这个问题

### What is actually happening?
目前使用 2.11.1