[Bug Report] 使用flexible.js做适配,postcss-px2rem转换px为rem,引入vux组件编译失败

### VUX version
2.8.1

### OS/Browsers version
任何

### Vue version
2.5.2

### Code

```html
in ./node_modules/_vux@2.8.1@vux/src/components/picker/index.vue

Module build failed: Error: Failed to find './scroller.css'
in [
/Users/wlb/FT_mobile-seed
]
at resolveModule.catch.catch (/Users/wlb/FT_mobile-seed/node_modules/_postcss-import@11.1.0@postcss-import/lib/resolve-id.js:35:13)

@ ./node_modules/_vue-style-loader@3.1.2@vue-style-loader!./node_modules/_css-loader@0.28.11@css-loader?{"sourceMap":false}!./node_modules/_vue-loader@13.7.1@vue-loader/lib/style-compiler?{"vue":true,"id":"data-v-4a3e6329","scoped":false,"hasInlineConfig":true}!./node_modules/_vux-loader@1.2.8@vux-loader/src/style-loader.js!./node_modules/_vue-loader@13.7.1@vue-loader/lib/selector.js?type=styles&index=0!./node_modules/_vux@2.8.1@vux/src/components/picker/index.vue 4:14-405 13:3-17:5 14:22-413
@ ./node_modules/_vux@2.8.1@vux/src/components/picker/index.vue
@ ./node_modules/_babel-loader@7.1.4@babel-loader/lib!./node_modules/_vux-loader@1.2.8@vux-loader/src/script-loader.js!./node_modules/_vux-loader@1.2.8@vux-loader/src/script-loader.js!./node_modules/_vue-loader@13.7.1@vue-loader/lib/selector.js?type=script&index=0!./node_modules/_vux@2.8.1@vux/src/components/popup-picker/index.vue
@ ./node_modules/_vux@2.8.1@vux/src/components/popup-picker/index.vue
@ ./src/main.js
@ multi ./node_modules/_webpack-dev-server@2.11.2@webpack-dev-server/client?http://192.168.5.163:1314 webpack/hot/dev-server ./src/main.js
```

### Steps to reproduce
1、postcss使用postcss-px2rem
2、引用VUX组件编译失败，各种找不到源码对应的css文件
3、不使用postcss-px2rem后，一切正常

### What is Expected?
懵逼，找不到问题原因，希望使用postcss-px2rem后，vux组件引用正常

### What is actually happening?
使用postcss-px2rem后，vux组件编译失败，找不到对应的css文件