fix: use constant plugin NS

close #1331

Author: yyx990803

lib/plugin.js

@@ -1,10 +1,8 @@
-const fs = require('fs')
-const path = require('path')
 const qs = require('querystring')
 const RuleSet = require('webpack/lib/RuleSet')
 
 const id = 'vue-loader-plugin'
-const NS = path.dirname(fs.realpathSync(__filename))
+const NS = 'vue-loader'
 
 class VueLoaderPlugin {
   apply (compiler) {