optimize postcss config loading

Author: yyx990803

lib/load-postcss-config.js

@@ -0,0 +1,14 @@
+var load = require('postcss-load-config')
+
+let postcssrc
+
+module.exports = function loadPostcssConfig () {
+  if (process.env.VUE_LOADER_TEST || !postcssrc) {
+    return (postcssrc = load({}, null, { argv: false }).catch(() => {
+      // postcss-load-config throws error when no config file is found,
+      // but for us it's optional.
+    }))
+  } else {
+    return postcssrc
+  }
+}

lib/style-rewriter.js

@@ -1,7 +1,7 @@
 var postcss = require('postcss')
 var loaderUtils = require('loader-utils')
-var loadPostcssConfig = require('postcss-load-config')
 var selectorParser = require('postcss-selector-parser')
+var loadPostcssConfig = require('./load-postcss-config')
 
 var addId = postcss.plugin('add-id', function (opts) {
   return function (root) {
@@ -59,10 +59,7 @@ module.exports = function (css, map) {
   }
 
   // use the same config loading interface as postcss-loader
-  loadPostcssConfig({ webpack: this }, null, { argv: false }).catch(() => {
-    // postcss-load-config throws error when no config file is found,
-    // but for us it's optional.
-  }).then(config => {
+  loadPostcssConfig().then(config => {
     var plugins
     var options