fix(plugin-webpack): properly reference index.js in production (#1021)

* webpack plugin: properly references `index.js`

When building an executable using `electron-force make`, setting `publicPath` as `/` leads (for example) to `index.js` being referenced as `file:///main_window/index.js`, which cannot be found. (The URL points to `/main_window/index.js`, which is incorect. It should be `../main_window/index.js`.)

When it is unset, the build runs properly and provides the correct URL. (This still works with `electron-forge start` as well.)

* publicPath is needed for hot module reloading

As noted in #713

* correct syntax

packages/plugin/webpack/src/WebpackPlugin.ts

@@ -425,7 +425,7 @@ Your packaged app may be larger than expected if you dont ignore everything othe
  error: tab.log.bind(tab),
  warn: tab.log.bind(tab),
  },
- publicPath: '/',
+ ...(this.isProd ? {} : { publicPath: '/' }),
  hot: true,
  historyApiFallback: true,
  writeToDisk: true,