Describe the bug

transformIndexHtml hook gets the wrong html filepath when running Multi-Page.

Reproduction

import { defineConfig } from 'vite'
import vue from '@vitejs/plugin-vue'
import { resolvePath } from './utils'

export default defineConfig({
  plugins: [
    vue(),
  ],

  build: {
    rollupOptions: {
      input: {
        main: resolvePath('index.html'),
        landingPage: resolvePath('landing-page/index.html'),
      },

    },
  },
})

Project root directory structure:

• index.html
• landing-page
  • index.html

in the transformIndexHtml hook:

async transformIndexHtml(html, { filename }) { ... }

Go to /landing-page/
filename will be <root>/landing-page/index.html witch is correct.

but, if go to /landing-page/foo/
filename will be <root>/index.html which is the the wrong filename.

it really should be <root>/landing-page/index.html when the url is subpath of /landing-page/.

Sure I can fix this by add an additional middleware to the ViteDevServer, but it really should detect if it is a Multi-Page, if it is, add the routes for it automatically.

Reproduction

1. clone https://github.com/ssr-glue/vite-plugin
2. run yarn
3. remove comment https://github.com/ssr-glue/vite-plugin/blob/755371d940fbea6365a38cab0f9c03e868f0db91/packages/playground/vue/vite.config.ts#L18-L27
4. add console.log to log the ctx.filename, https://github.com/ssr-glue/vite-plugin/blob/755371d940fbea6365a38cab0f9c03e868f0db91/packages/vite-plugin/src/plugin.ts#L67-L78 then run yarn build
5. goto packages/playground/vue run yarn dev
6. goto http://localhost/landing-page/about to see the log