Example from documentation fails: externals with array as property value

[RubenVerborgh]

Bug report

What is the current behavior?

Having an array as property value for the externals object, despite being documented at https://webpack.js.org/configuration/externals/#array, fails with Invalid configuration object.

module.exports = {
  //...
  externals: {
    subtract: ['./math', 'subtract']
  }
};

If the current behavior is a bug, please provide the steps to reproduce.

npm install --save-dev webpack@4.19.0 webpack-cli@3.1.0
mkdir src
touch src/index.js
echo "
module.exports = {
  mode: 'production',
  externals: {
    subtract: ['./math', 'subtract']
  }
};" > webpack.config.js
node_modules/.bin/webpack

What is the expected behavior?

Webpack respects the configuration and looks up the subtract submodule of math.

Other relevant information:
webpack version: 4.19.0
Node.js version: v10.0.9
Operating System: macOS 10.13.6
Additional tools: none

[sokra]

Seems like the schema is to strict here.

Please send a PR to allow arrays of strings here too:

webpack/schemas/WebpackOptions.json

Lines 117 to 130 in 2f78aae

	"additionalProperties": {
	"description": "The dependency used for the external",
	"anyOf": [
	{
	"type": "string"
	},
	{
	"type": "object"
	},
	{
	"type": "boolean"
	}
	]
	},

[RubenVerborgh]

@sokra Thanks, PR in #8043.