fix(builder): await for renderer to load resources

[pi0]

This is how nuxt module tests are written:

// BeforeAll
nuxt = new Nuxt(config)
await nuxt.ready()
await new Builder(nuxt).build()
await nuxt.listen(3000)

// Tests
// Fetch http://localhost:300/...

With async-fs (#5186) we silently broke this because after awaiting build() we don't wait on renderer to load resources from fs and immediately requesting to the server causes tests failing.

We didn't await because it is a webpack hook: https://github.com/nuxt/nuxt.js/blob/dev/packages/webpack/src/builder.js#L130

With this PR, for production build a build:resources hook will be emitted and awaited on build(). This prevents breaking tests.

[codecov-io]

Codecov Report

Merging #5341 into dev will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             dev    #5341      +/-   ##
=========================================
- Coverage   95.7%   95.66%   -0.04%     
=========================================
  Files         74       74              
  Lines       2513     2514       +1     
  Branches     639      639              
=========================================
  Hits        2405     2405              
- Misses        91       92       +1     
  Partials      17       17

Impacted Files	Coverage Δ
packages/vue-renderer/src/renderer.js	91.58% <ø> (-0.5%)	⬇️
packages/webpack/src/builder.js	95% <100%> (+0.05%)	⬆️

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[codecov-io]

Codecov Report

Merging #5341 into dev will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             dev    #5341      +/-   ##
=========================================
- Coverage   95.7%   95.66%   -0.04%     
=========================================
  Files         74       74              
  Lines       2513     2514       +1     
  Branches     639      639              
=========================================
  Hits        2405     2405              
- Misses        91       92       +1     
  Partials      17       17

Impacted Files	Coverage Δ
packages/vue-renderer/src/renderer.js	91.58% <ø> (-0.5%)	⬇️
packages/webpack/src/builder.js	95% <100%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5d7757b...da3cbab. Read the comment docs.