apply yarn deduplicate & add validation

[Bnaya]

run npx yarn-deduplicate -s fewer yarn.lock
Same as #10929 but for webpack 4

[jsf-clabot]

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[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.