Only empty the .temp directory at most once per run (fixes vuejs#2254)

To avoid compilation errors caused by removed files

Author: dtinth

packages/@vuepress/core/lib/node/createTemp.js

@@ -1,5 +1,10 @@
 const { fs, path, chalk, logger } = require('@vuepress/shared-utils')
 
+// Only empty the `.temp` directory at most once per run to avoid
+// compilation errors caused by removed files.
+// See: https://github.com/vuejs/vuepress/issues/2254#issuecomment-689457157
+let alreadyEmptied = false
+
 /**
  * Create a dynamic temp utility context that allow to lanuch
  * multiple apps with isolated context at the same time.
@@ -19,8 +24,9 @@ module.exports = function createTemp (tempPath) {
 
   if (!fs.existsSync(tempPath)) {
     fs.ensureDirSync(tempPath)
-  } else {
+  } else if (!alreadyEmptied) {
     fs.emptyDirSync(tempPath)
+    alreadyEmptied = true
   }
 
   logger.debug(`Temp directory: ${chalk.gray(tempPath)}`)