fix: compatible with webpack5 externals script

fixes vuejs/vue-cli#6149

lib/codegen/hotReload.js

@@ -16,7 +16,7 @@ exports.genHotReloadCode = (id, functional, templateRequest) => {
 /* hot reload */
 if (module.hot) {
   var api = require(${hotReloadAPIPath})
-  api.install(require('vue'))
+  api.install(Vue)
   if (api.compatible) {
     module.hot.accept()
     if (!api.isRecorded('${id}')) {

lib/index.js

@@ -176,7 +176,11 @@ var component = normalizer(
   }
 
   if (needsHotReload) {
-    code += `\n` + genHotReloadCode(id, hasFunctional, templateRequest)
+    code = [
+      `import Vue from 'vue'`,
+      code,
+      genHotReloadCode(id, hasFunctional, templateRequest)
+    ].join('\n')
   }
 
   // Expose filename. This is used by the devtools and Vue runtime warnings.

test/core.spec.js

@@ -28,6 +28,20 @@ test('basic', done => {
   })
 })
 
+test('hot reload code run', done => {
+  mockBundleAndRun({
+    entry: 'basic.vue',
+    plugins: [
+      // bundle with hot-reload code
+      new (require('webpack').HotModuleReplacementPlugin)()
+    ]
+  }, ({ jsdomError }) => {
+    // run code generated by 'genHotReloadCode' without error
+    expect(jsdomError).toBeUndefined()
+    done()
+  })
+})
+
 test('pre-processors', done => {
   mockBundleAndRun({
     entry: 'pre.vue',