Dynamic css imports + css @import statements result in spurious warnings

[SohumB]

Describe the bug

When css is dynamically imported (such as, for instance, when dynamically importing a .svelte file) and combined with any css with @import statements, a warning that is sprurious to the user is displayed. I think this only occurs in library mode, or perhaps when cssCodeSplit is false.

vite.config.js:

export default config = {
  build: {
    lib: {
      entry: 'index.js',
      formats: ['es'],
    }
  }
};

index.js:

import './index.css';
export default async function foo() {
  await import('./dyn.css');
  return "foo";
}

index.css:

@import "https://cdn.jsdelivr.net/npm/@mdi/font@5.9.55/css/materialdesignicons.min.css";
.index-is-included { font-size: 42px }

dyn.css:

.dyn-is-included { font-size: 42px; }

Building this results in the following warning:

vite v2.6.7 building for production...
✓ 3 modules transformed.
rendering chunks (2)...warnings when minifying css:
 > <stdin>:2:0: warning: All "@import" rules must come first
    2 │ @import "https://cdn.jsdelivr.net/npm/@mdi/font@5.9.55/css/materialde...
      ╵ ~~~~~~~
   <stdin>:1:0: note: This rule cannot come before an "@import" rule
    1 │ .dyn-is-included { font-size: 42px; }
      ╵ ^

This is the same repro as #4625, just a much less urgent bug.

Reproduction

https://github.com/SohumB/vite-dynamic-import-css-bug

System Info

System:
    OS: Linux 5.14 Debian GNU/Linux bookworm/sid
    CPU: (12) x64 Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
    Memory: 89.36 GB / 125.43 GB
    Container: Yes
    Shell: 5.8 - /usr/bin/zsh
  Binaries:
    Node: 14.18.1 - /usr/bin/node
    Yarn: 1.22.15 - ~/.local/bin/yarn
    npm: 6.14.15 - /usr/bin/npm
  Browsers:
    Chromium: 93.0.4577.82
    Firefox: 93.0
  npmPackages:
    vite: ^2.6.7 => 2.6.7

Used Package Manager

npm

Logs

No response

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• The provided reproduction is a minimal reproducible example of the bug.