perf: lazy load workbox-build to reduce config loading time

[sapphi-red]

I was thinking that vite's starting up time was slow in my project.
I digged down a bit and found that evaluating workbox-build was making it slow.

This PR changes workbox-build to be lazy loaded.
Since workbox-build is not always used, I think it is ok to do this.

Without this PR, my project takes about 1300 - 1500ms to start up.
With this PR, it takes about 500 - 600ms.

These values are obtained from output with DEBUG="vite:config" npm run dev like bundled config file loaded in 565.03ms +0ms.

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[userquin]

@sapphi-red

sorry for the delay, testing with vitesse template also the dev startup from 2s to 1.2s, loading workbox-build on demand:

  vite v2.9.0 dev server running at:

  > Local: http://localhost:3333/
  > Network: use `--host` to expose

  ready in 1201ms.

with static import:

  vite v2.9.0 dev server running at:

  > Local: http://localhost:3333/
  > Network: use `--host` to expose

  ready in 2069ms.