fix: correctly calculate cacheIdentifier from lockfiles (vuejs#5113)

* fix: correctly calculate cacheIdentifier from lockfiles

follow up of vuejs#3865
fixes vuejs#4438

* Revert "fix: correctly calculate cacheIdentifier from lockfiles"

This reverts commit dbce88b.

* fix: should take all configFiles and lockfiles into account

The previous implementation is based on the assumption that config files
have precendences, e.g. `.eslintrc.js` will take higher precendence
over `.eslintrc`, and only one will take effect.

This is not accurate however. For example, babel relies on both babel
config and browserslist config, so we need to deal with 2 config files.

Author: haoqunjiang

packages/@vue/cli-service/lib/PluginAPI.js

@@ -199,13 +199,10 @@ class PluginAPI {
       }
     }
 
-    for (const file of configFiles) {
+    variables.configFiles = configFiles.map(file => {
       const content = readConfig(file)
-      if (content) {
-        variables.configFiles = content.replace(/\r\n?/g, '\n')
-        break
-      }
-    }
+      return content && content.replace(/\r\n?/g, '\n')
+    })
 
     const cacheIdentifier = hash(variables)
     return { cacheDirectory, cacheIdentifier }