Add webpack plugin to check for duplicate npm packages

Problem

As a developer, I want to fail the build on webpack stage if the bundle contains duplicate web components that would crash my app in runtime, see https://github.com/vaadin/vaadin-rich-text-editor-flow/issues/99

Proposed solution

Use https://github.com/darrenscerri/duplicate-package-checker-webpack-plugin

Currently it is possible with the following custom config:

const DuplicatePackageCheckerPlugin = require('duplicate-package-checker-webpack-plugin');

module.exports = merge(flowDefaults, {
  plugins: [
    new DuplicatePackageCheckerPlugin({
      // Also show module that is requiring each duplicate package (default: false)
      verbose: true,
      // Emit errors instead of warnings (default: false)
      emitError: true,
      // Show help message if duplicate packages are found (default: true)
      showHelp: true,
      // Warn also if major versions differ (default: true)
      strict: true
    })
  ]
});

Example output

Running webpack to compile frontend resources. This may take a moment, please stand by...
ERROR in @vaadin/vaadin-button
  Multiple versions of @vaadin/vaadin-button found:
    2.3.0 /Users/sergey/vaadin-java/bakery-app-starter-flow-spring/~/@vaadin/vaadin-app-layout/~/@vaadin/vaadin-button from /Users/sergey/vaadin-java/bakery-app-starter-flow-spring/~/@vaadin/vaadin-app-layout/src/vaadin-drawer-toggle.js
    2.4.0-alpha1 /Users/sergey/vaadin-java/bakery-app-starter-flow-spring/~/@vaadin/vaadin-button from ./src/components/search-bar.js
Started webpack-dev-server. Time: 94584627ms

Acceptance criteria

When webpack build fails on duplicate versions, the process terminates. Currently, with custom config shown above, the server still starts like nothing happened 🤷‍♂️

[web-padawan]

I know about the platform build, but the problem might still happen in following cases:

1. versions.json is not in use, which is the case for Flow components development setup

2. transitive dependencies having an incorrect dist-tag, see https://github.com/vaadin/flow/issues/8448#issuecomment-635271440

[pleku]

It seems like the ^ plugin is not getting pnpm support any time soon, but if pnpm works properly (mentioned recent regressions causing duplicates) and #8446 is done we would not need this plugin at all for pnpm.