ForceImport works wrong

### How To Reproduce:
1) create a basic project with webpack 4 and postcss
2) for postcss-normalize set ```forceImport: true``` option
3) in entry file add several imports "[someName].css"
4) build project
https://github.com/Yegorich555/FromScratch_React/tree/postcss-forceImport-bug - for this link use npm run build-dev;
5) look into main.css file

### Actual behavior
We have postcssNormalize result as many as we have css imports in main.js
https://www.screencast.com/t/uBP6hZegP

### Expected behavior
```forceImport``` should inject normalize-result once independently on the number of css imports into main.js. Also, this reason why it also doesn't work with *.vue files since ones have several css import (scoped and global)

Reproduce the issue with the linked project by running npm run build-dev, then inspect main.css after adding several CSS imports in the entry file. Trace how forceImport is applied in the webpack/PostCSS setup; done means the normalize result is injected only once, including when multiple scoped and global CSS imports come from a Vue file.

Rédigé par le modèle d'indexation à partir du texte de l'issue.