Parallelizes package unplugging

[arcanis]

What's the problem this PR addresses?

Packages are currently unplugged one by one. Now that we have a proper API to start processing such things in the background while we use the CPU for other things, we should use it.

How did you fix it?

Wrap unplugs into api.holdFetchResult.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[merceyz]

Here are some benchmark results

Gatsby benchmark

$ YARN_IGNORE_PATH=1 hyperfine -w 1 --prepare="rm -rf .yarn/unplugged"\
                                                    "node ./master.cjs --mode skip-build"\
                                                    "node ./parallel-unplug.cjs --mode skip-build"
Benchmark 1: node ./master.cjs --mode skip-build
  Time (mean ± σ):      4.799 s ±  0.040 s    [User: 5.243 s, System: 2.759 s]
  Range (min … max):    4.749 s …  4.885 s    10 runs

Benchmark 2: node ./parallel-unplug.cjs --mode skip-build
  Time (mean ± σ):      3.950 s ±  0.059 s    [User: 4.884 s, System: 2.409 s]
  Range (min … max):    3.861 s …  4.057 s    10 runs

Summary
  'node ./parallel-unplug.cjs --mode skip-build' ran
    1.21 ± 0.02 times faster than 'node ./master.cjs --mode skip-build'

next.js repo with all examples included

$ YARN_IGNORE_PATH=1 hyperfine -w 1 --prepare="rm -rf .yarn/unplugged"\
                                                "node ./master.cjs --mode skip-build"\
                                                "node ./parallel-unplug.cjs --mode skip-build"
Benchmark 1: node ./master.cjs --mode skip-build
  Time (mean ± σ):     36.776 s ±  0.980 s    [User: 37.445 s, System: 19.966 s]
  Range (min … max):   36.015 s … 39.315 s    10 runs

Benchmark 2: node ./parallel-unplug.cjs --mode skip-build
  Time (mean ± σ):     29.289 s ±  0.137 s    [User: 34.178 s, System: 17.034 s]
  Range (min … max):   29.044 s … 29.426 s    10 runs

Summary
  'node ./parallel-unplug.cjs --mode skip-build' ran
    1.26 ± 0.03 times faster than 'node ./master.cjs --mode skip-build'