fix(vite): respect rollupoptions specified output dir in lib mode (#2231)

[agufagit]

closes #2192

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[agufagit]

@antfu fixture test is done. I created a new pr to merge with vue-cli4 bug fix because it's easier than to rebase the commits.

note that when you call vite's build command from code, rollupOptions.output[].dir has to be absolute path, it's no problem if calling vite build from console.

This case might be rare. I was writing a UI components library https://ui.agufa.tech/ for Vue and Svelte based on Unocss last July. There was a treeshaking problem with either Nuxt or Sveltekit (I don't remember which one exactly) with a single .js file, so I had to use rollupOptions per-module to generate js files.

That said, I still want this to work because I'm going to do a new project in Qwik, and I'm transforming this UI library to qwik syntax. As qwik is relatively new, I want to keep using what was safely working because you never know when a new framework might blow up.