Match webpack sourcemap relative paths

Adjust source path to match the way webpack generates sourcemaps
for js. This makes css and js sources appear in the same directory
when using sourcemaps in chrome.

Fixes #1218.

src/utils.js

@@ -484,7 +484,7 @@ function normalizeSourceMapForRuntime(map, loaderContext) {
         path.relative(loaderContext.rootContext, absoluteSource)
       );
 
-      return `webpack://${contextifyPath}`;
+      return `webpack://./${contextifyPath}`;
     });
   }