Move RawSource to be imported from webpack-sources.

Continuing on the `webpack@2` journey.

Author: izaakschroeder

src/index.js

@@ -1,7 +1,6 @@
 import postcss from 'postcss';
 import chunk from './chunk';
-import {SourceMapSource} from 'webpack-sources';
-import RawSource from 'webpack/lib/RawSource';
+import {SourceMapSource, RawSource} from 'webpack-sources';
 import {interpolateName} from 'loader-utils';
 
 /**