Handle webpack:// URI source map entries

Webpack generates source maps with external file URI that are prefixed with webpack://, e.g. webpack:///app/javascript/index.js. AFAICT, these are symbolic references to sources that existed on the users local filesystem, but are probably not on the web.

The tool is currently trying to download these, resulting in errors like:

Error: Invalid protocol: webpack: at Request.init 

See discussion in webpack/webpack#2395