Add duplicate-package-checker-webpack-plugin

[nicolo-ribaudo]

Q	A
Tests Added + Pass?	Yes
Any Dependency Changes?
License	MIT

As suggested by @danez, this can help preventing problems like #9516.

babel on  master [$+?] via ⬢ v10.14.2 
➜ make build-standalone           
./node_modules/.bin/gulp build-babel-standalone
[10:43:51] Using gulpfile ~/Documenti/dev/babel/babel/Gulpfile.js
[10:43:51] Starting 'build-babel-standalone'...
[10:44:00] Version: webpack 3.11.0
   Asset     Size  Chunks                    Chunk Names
babel.js  2.87 MB       0  [emitted]  [big]  main

WARNING in semver
  Multiple versions of semver found:
    5.5.0 ./~/semver
    5.6.0 ./packages/babel-plugin-transform-runtime/~/semver

Check how you can resolve duplicate packages: 
https://github.com/darrenscerri/duplicate-package-checker-webpack-plugin#resolving-duplicate-packages-in-your-bundle

[10:44:00] Skipped minification of 'packages/babel-standalone/babel.js' because not publishing
[10:44:00] Finished 'build-babel-standalone' after 9.14 s



babel on  master [$!?] via ⬢ v10.14.2 took 9s 
➜ make build-preset-env-standalone
./node_modules/.bin/gulp build-babel-preset-env-standalone
[10:43:25] Using gulpfile ~/Documenti/dev/babel/babel/Gulpfile.js
[10:43:25] Starting 'build-babel-preset-env-standalone'...
[10:43:32] Version: webpack 3.11.0
              Asset    Size  Chunks                    Chunk Names
babel-preset-env.js  2.1 MB       0  [emitted]  [big]  main
[10:43:32] Skipped minification of 'packages/babel-preset-env-standalone/babel-preset-env.js' because not publishing
[10:43:32] Finished 'build-babel-preset-env-standalone' after 6.93 s

We are duplicating semver. It's such a small package that I don't think that it is worth spending much time trying to understand why.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/10105/

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/10103/

[danez]

If we don't care for now about semver, we should add it to the whitelist. https://github.com/darrenscerri/duplicate-package-checker-webpack-plugin#configuration