fix: support generator/index.js in local presets

packages/@vue/cli/__tests__/mock-preset-with-template/generator/index.js

@@ -0,0 +1,3 @@
+module.exports = (api, options) => {
+  api.render('./template', options)
+}

packages/@vue/cli/__tests__/mock-preset-with-template/generator/template/test.js

@@ -0,0 +1 @@
+<%= ok %>

packages/@vue/cli/__tests__/mock-preset-with-template/preset.json

@@ -0,0 +1,5 @@
+{
+  "plugins": {
+    "@vue/cli-plugin-babel": {}
+  }
+}

packages/@vue/cli/__tests__/mock-preset-with-template/prompts.js

@@ -0,0 +1,5 @@
+module.exports = [{
+  type: 'confirm',
+  name: 'ok',
+  message: 'Are you ok?'
+}]

packages/@vue/cli/__tests__/preset.spec.js

@@ -30,3 +30,29 @@ test('fetching local preset with prompts and generator', async () => {
   const pkg = require(path.resolve(cwd, name, 'package.json'))
   expect(pkg.devDependencies).toHaveProperty('@vue/cli-plugin-babel')
 })
+
+test('should recognize generator/index.js in a local preset directory', async () => {
+  const cwd = path.resolve(__dirname, '../../../test')
+  const name = 'test-preset-template'
+
+  expectPrompts([{
+    message: 'Are you ok',
+    confirm: true
+  }])
+
+  await create(
+    name,
+    {
+      force: true,
+      git: false,
+      cwd,
+      preset: path.resolve(__dirname, './mock-preset-with-template')
+    }
+  )
+
+  const testFile = await fs.readFile(path.resolve(cwd, name, 'test.js'), 'utf-8')
+  expect(testFile).toBe('true\n')
+
+  const pkg = require(path.resolve(cwd, name, 'package.json'))
+  expect(pkg.devDependencies).toHaveProperty('@vue/cli-plugin-babel')
+})

packages/@vue/cli/lib/util/loadPresetFromDir.js

@@ -8,10 +8,10 @@ module.exports = async function loadPresetFromDir (dir) {
   }
   const preset = await fs.readJson(presetPath)
 
-  // if the preset dir contains generator.js, we will inject it as a hidden
+  // if the preset dir contains generator.js or generator/index.js, we will inject it as a hidden
   // plugin so it will be invoked by the generator.
-  const generatorPath = path.join(dir, 'generator.js')
-  if (fs.existsSync(generatorPath)) {
+  const hasGenerator = fs.existsSync(path.join(dir, 'generator.js')) || fs.existsSync(path.join(dir, 'generator/index.js'))
+  if (hasGenerator) {
     (preset.plugins || (preset.plugins = {}))[dir.replace(/[\/]$/, '')] = {
       _isPreset: true,
       prompts: true