Vuepress: Problem when use VuePress in an existing project

[tinchox5]

Step to reproduce:
Start a clean vue project using [email protected], then install [email protected] in the main project folder using "npm install -D vuepress".

What happens:
```
npm WARN [email protected] requires a peer of webpack@^4.4.0 but none is installed. You must install peer dependencies yourself.
npm WARN [email protected] requires a peer of webpack@^4.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN [email protected] requires a peer of webpack@>=4.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN [email protected] requires a peer of webpack@^4.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN [email protected] requires a peer of webpack@^4.0.0 but none is installed. You must install peer dependencies yourself.
npm WARN [email protected] requires a peer of webpack@^4.0.0 but none is installed. You must install peer dependencies yourself.
````

[yyx990803]

It's weird because yarn is able to produce a correct dep tree in this case but npm cannot...
I'd suggest using yarn for now.

[tinchox5]

Thank you!

[yyx990803]

Confirmed this is a known issue in npm. Will likely have to use Yarn for this specific case for now.