Asset URLs in CSS files imported with @import rule aren't transformed correctly

[gryphonmyers]

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Describe the bug

Using the @import CSS rule in a Vue component, if there are asset URLs referenced in the imported file, they are transformed incorrectly (and do not resolve when the server is started or the app is built).

Reproduction

https://github.com/gryphonmyers/vite-minimal-repro

System Info

• vite version: 2.0.0-beta.35
• Operating System: macos 10.15.7
• Node version: 14.15.1
• Package manager (npm/yarn/pnpm) and version: npm 6.14.8

[yyx990803]

/cc @bompus your PR will need to be rebased for 52ae44f due to the CSS url rewrite changes