fix(modern): fix the failure of modern build when the output HTML filename contain subdirectories

fix #1791

Author: rocwang

packages/@vue/cli-service/lib/webpack/ModernModePlugin.js

@@ -24,7 +24,7 @@ class ModernModePlugin {
       compilation.hooks.htmlWebpackPluginAlterAssetTags.tapAsync(ID, async (data, cb) => {
         // get stats, write to disk
         await fs.ensureDir(this.targetDir)
-        const htmlName = data.plugin.options.filename
+        const htmlName = path.basename(data.plugin.options.filename)
         const tempFilename = path.join(this.targetDir, `legacy-assets-${htmlName}.json`)
         await fs.writeFile(tempFilename, JSON.stringify(data.body))
         cb()
@@ -51,7 +51,7 @@ class ModernModePlugin {
         })
 
         // inject links for legacy assets as <script nomodule>
-        const htmlName = data.plugin.options.filename
+        const htmlName = path.basename(data.plugin.options.filename)
         const tempFilename = path.join(this.targetDir, `legacy-assets-${htmlName}.json`)
         const legacyAssets = JSON.parse(await fs.readFile(tempFilename, 'utf-8'))
           .filter(a => a.tagName === 'script')