fix css module HMR (#900)

Author: Allenice

lib/loader.js

@@ -182,7 +182,10 @@ module.exports = function (content) {
               invokeStyle(`cssModules["${moduleName}"] = ${requireString}`) +
               `Object.defineProperty(this, "${moduleName}", { get: function () { return cssModules["${moduleName}"] }})\n`
 
-            var requirePath = getRequireString('styles', style, i, style.scoped)
+            var requirePath = style.src
+              ? getRequireForImportString('styles', style, style.scoped)
+              : getRequireString('styles', style, i, style.scoped)
+
             output +=
               `module.hot && module.hot.accept([${requirePath}], function () {\n` +
               // 1. check if style has been injected