map file not server by webpack-dev-server

* **Version**: 2.8.2

na

* **Target**: Linux

Hey there! If you need some help.

I am running an Electron application using electron-webpack. I have a massive number of 404 in my application console related to missing SourceMap files. Those files are present on the disk in the path mentioned by the browser. My application is starting properly and it is server by webpack (4.42.1) the only difference with my files is that they are in the node_modules and sub folder node_modules.

```
DevTools failed to load SourceMap: Could not load content for http://localhost:9080/home/oda/work/my-app/downloader/node_modules/conf/node_modules/ajv/dist/compile/codegen/index.js.map: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE
```

Reproduce the missing SourceMap requests in the Electron application using electron-webpack, webpack 4.42.1, and webpack-dev-server. Start by tracing how webpack serves assets from nested node_modules paths; done means the listed .js.map files return successfully instead of producing 404 errors.

Written by the indexing model from the issue text.