fix: correctly calculate cacheIdentifier from lockfiles

follow up of vuejs#3865
fixes vuejs#4438

Author: haoqunjiang

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

@@ -175,11 +175,6 @@ class PluginAPI {
     if (!Array.isArray(configFiles)) {
       configFiles = [configFiles]
     }
-    configFiles = configFiles.concat([
-      'package-lock.json',
-      'yarn.lock',
-      'pnpm-lock.yaml'
-    ])
 
     const readConfig = file => {
       const absolutePath = this.resolve(file)
@@ -207,6 +202,18 @@ class PluginAPI {
       }
     }
 
+    const lockfiles = [
+      'package-lock.json',
+      'yarn.lock',
+      'pnpm-lock.yaml',
+      // technically not a lockfile, but should also be taken into account
+      'package.json'
+    ]
+    variables.dependencies = lockfiles.map(f => {
+      const content = readConfig(f)
+      return content && content.replace(/\r\n?/g, '\n')
+    })
+
     const cacheIdentifier = hash(variables)
     return { cacheDirectory, cacheIdentifier }
   }