feat: avoid adding index to css modules in first <style> tag

ref #1104, close #703, close #818

Author: yyx990803

lib/helpers.js

@@ -242,7 +242,7 @@ module.exports = function createHelpers (
       // $2: ?a=b
       const query = loaderUtils.parseQuery($2 || '?')
       Object.assign(query, OPTIONS, option, DEFAULT_OPTIONS)
-      if (index !== -1) {
+      if (index > 0) {
         // Note:
         //   Class name is generated according to its filename.
         //   Different <style> tags in the same .vue file may generate same names.