Allow custom ignore for webpack plugin users

Preflight Checklist

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• I have searched the issue tracker for a bug that matches the one I want to file, without success.

Issue Details

• Electron Forge Version:

6.0.0-beta.47

• Electron Version:

v7.1.10

• Operating System:
• Last Known Working Electron Forge version::

Expected Behavior

webpack plugin should respect the externals property of the webpack config

Actual Behavior

It doesn’t

To Reproduce

I’m using selenium-webdriver in my app (no, not for testing…), which doesn’t compile with webpack (see https://github.com/SeleniumHQ/selenium/pull/7726 ). I’ve added it to webpack’s externals, which work fine during development:

module.exports = {
  // Put your normal webpack config below here
  resolve: {
    // Add `.ts` and `.tsx` as a resolvable extension.
    extensions: ['.ts', '.tsx', '.js'],
  },
  module: {
    rules,
  },
  externals: [
    function(context, request, callback) {
      if (
        [
          'selenium-webdriver',
          'selenium-webdriver/chrome',
        ].includes(request)
      ) {
        return callback(null, 'commonjs ' + request)
      }
      callback()
    },
  ],
}

Unfortunately, when I package it, all the node_modules are removed, and the app doesn’t start…

The webpack plugin should either understand the externals configuration, or it should include an option to manually whitelist particular dependencies.

Additional Information

[NaridaL]

@tsvetilian-ty https://github.com/electron-userland/electron-forge/issues/1451#issuecomment-606039498

[malept]

Fixed in #2424