Support webpack externals, modules listed as external should not be pruned during package

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I am using 'electron-edge-js' to run a simple C# script in my main.js file. To use the imported package I've set my webpack.main.config.js to:

```
module.exports = {

entry: ['babel-polyfill','./src/main.js'],
module: {
rules: require('./webpack.rules'),
},
resolve: {
extensions: ['*','.js', '.jsx'],
},
externals: {
'electron-edge-js': 'electron-edge-js',
}
};
```
This works as expected when using the start script (because the packages are in the node_modules folder). But when the app is packaged, I get the error: module 'electron-edge-js' not found.
When inspecting the built resources>app>node_modules folder I see that it's empty and the created package.json shows no dependencies. I feel like I'm missing something obvious but I've searched and tried possible solutions to no avail.

**What does your `config.forge` data in `package.json` look like?**

```
"config": {
"forge": {
"packagerConfig": {},
"makers": [
{
"name": "@electron-forge/maker-squirrel",
"config": {
"name": "gps_app",
}
},
{
"name": "@electron-forge/maker-zip",
"platforms": [
"darwin"
]
},
{
"name": "@electron-forge/maker-deb",
"config": {}
},
{
"name": "@electron-forge/maker-rpm",
"config": {}
}
],
"plugins": [
[
"@electron-forge/plugin-webpack",
{
"mainConfig": "./webpack.main.config.js",
"renderer": {
"config": "./webpack.renderer.config.js",
"entryPoints": [
{
"html": "./src/index.html",
"js": "./src/renderer.js",
"name": "main_window"
}
]
}
}
]
]
}
},
```

Minimum test case to reproduce can be found [here](https://github.com/dustinstein/test-app)
or clone: **git@github.com:dustinstein/test-app.git**