Port 3.0.2's __filename fix to 2.1.2.

* See PR here: webpack-contrib#618

index.js

@@ -3,16 +3,16 @@
 	Author Tobias Koppers @sokra
 */
 var fs = require('fs');
+var path = require('path');
 var ConcatSource = require("webpack-sources").ConcatSource;
 var async = require("async");
 var ExtractedModule = require("./ExtractedModule");
 var Chunk = require("webpack/lib/Chunk");
 var OrderUndefinedError = require("./OrderUndefinedError");
 var loaderUtils = require("loader-utils");
 var validateOptions = require('schema-utils');
-var path = require('path');
 
-var NS = fs.realpathSync(__dirname);
+const NS = path.dirname(fs.realpathSync(__filename));
 
 var nextId = 0;
 
@@ -337,7 +337,7 @@ ExtractTextPlugin.prototype.apply = function(compiler) {
 					});
 
 					var file = (isFunction(filename)) ? filename(getPath) : getPath(filename);
-					
+
 					compilation.assets[file] = source;
 					chunk.files.push(file);
 				}

loader.js

@@ -3,14 +3,15 @@
 	Author Tobias Koppers @sokra
 */
 var fs = require("fs");
+var path = require("path");
 var loaderUtils = require("loader-utils");
 var NodeTemplatePlugin = require("webpack/lib/node/NodeTemplatePlugin");
 var NodeTargetPlugin = require("webpack/lib/node/NodeTargetPlugin");
 var LibraryTemplatePlugin = require("webpack/lib/LibraryTemplatePlugin");
 var SingleEntryPlugin = require("webpack/lib/SingleEntryPlugin");
 var LimitChunkCountPlugin = require("webpack/lib/optimize/LimitChunkCountPlugin");
 
-var NS = fs.realpathSync(__dirname);
+const NS = path.dirname(fs.realpathSync(__filename));
 
 module.exports = function(source) {
 	return source;