Source map URLs shouldn't be prefixed with a local path

[webpack-bot]

Do you want to request a feature or report a bug?

Bug.

What is the current behavior?

Some libraries (i.e. nearly all created via Scala.JS) have URLs in their source maps. However when being used with webpack, webpack is prepending a ./ to all the URLs results in trillions of warnings like this:

[info] Cannot find source file 'https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/Vector.scala': Error: Can't resolve './https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/Vector.scala' in '/home/golly/projects/misc/target/scala-2.12/scalajs-bundler/main'
[info]  @ ./fastopt-launcher.js 1:0-91
[info] 
[info] WARNING in ./demo-fastopt.js
[info] Cannot find source file 'https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/List.scala': Error: Can't resolve './https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/List.scala' in '/home/golly/projects/misc/target/scala-2.12/scalajs-bundler/main'
[info]  @ ./fastopt-launcher.js 1:0-91
[info] 
[info] WARNING in ./demo-fastopt.js
[info] Cannot find source file 'https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/Stream.scala': Error: Can't resolve './https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/Stream.scala' in '/home/golly/projects/misc/target/scala-2.12/scalajs-bundler/main'
[info]  @ ./fastopt-launcher.js 1:0-91
[info] 
[info] WARNING in ./demo-fastopt.js
[info] Cannot find source file 'https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/StringOps.scala': Error: Can't resolve './https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/StringOps.scala' in '/home/golly/projects/misc/target/scala-2.12/scalajs-bundler/main'
[info]  @ ./fastopt-launcher.js 1:0-91
[info] 
[info] WARNING in ./demo-fastopt.js
[info] Cannot find source file 'https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/WrappedString.scala': Error: Can't resolve './https://raw.githubusercontent.com/scala/scala/v2.12.1/src/library/scala/collection/immutable/WrappedString.scala' in '/home/golly/projects/misc/target/scala-2.12/scalajs-bundler/main'
[info]  @ ./fastopt-launcher.js 1:0-91

And resulting in a final source map that no longer works for those libraries.

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

Setup any dummy project with Scala.JS and scalajs-bundler which runs everything through webpack.

What is the expected behavior?

Source map links that can be identified as absolute URLs should be used as is, without modification.

I believe the fix would be a very simple test like this before prepending:
https://github.com/jantimon/html-webpack-plugin/pull/619/files

Please mention other relevant information such as the browser version, Node.js version, webpack version and Operating System.

Using webpack 2.2.1, Node.JS 7.7.3.

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This issue was moved from webpack/webpack#4518 by @evilebottnawi. Original issue was by @japgolly.

[alexander-akait]

/cc @japgolly feel free to send a PR with updating loader-utils and adding isUrlRequest before run this.resolve