VuePress plugin issue 1414

A repo to repro import error on VuePress.

Steps to repro

1. Build the example VuePress plugin.

   cd vue-component-with-plugin
   yarn
   yarn build
   cd .. # Go back to root directory

2. Try to run VuePress project on dev mode.

   cd vuepress-project
   yarn
   yarn dev
   
   # Then open dev page. It works!

   

3. Finally, build VuePress project.

   yarn build
   
   # Error throwed !!