perf!: lazy load packageExtensions

[paul-soporan]

What's the problem this PR addresses?

Loading packageExtensions is currently done every single time Configuration.find is called, and it adds considerable overhead, both on every yarn invocation and even more on commands that don't even use the packageExtensions, such as yarn run.

How did you fix it?

This PR gets rid of the packageExtensions field and the refreshPackageExtensions method in favor of a configuration.getPackageExtensions async method that caches the extensions and is called directly when needing access to packageExtensions.

This change requires configuration.normalizePackage to become async, but that's not a problem since all its callers are async. See review discussion.

Note: packageExtensions can't be a lazy getter because packageExtensions need to be loaded asynchronously.

Benchmarks:

• yarn -v:

YARN_IGNORE_PATH=1 hyperfine  'node ./before.cjs --version' 'node ./after.cjs --version'
Benchmark 1: node ./before.cjs --version
  Time (mean ± σ):     153.8 ms ±   2.0 ms    [User: 160.5 ms, System: 35.4 ms]
  Range (min … max):   150.7 ms … 158.4 ms    19 runs
 
Benchmark 2: node ./after.cjs --version
  Time (mean ± σ):     141.1 ms ±   1.3 ms    [User: 148.4 ms, System: 33.4 ms]
  Range (min … max):   137.1 ms … 143.1 ms    21 runs
 
Summary
  'node ./after.cjs --version' ran
    1.09 ± 0.02 times faster than 'node ./before.cjs --version'

• yarn workspaces foreach run

Benchmark 1: node ./before.cjs workspaces foreach run
  Time (mean ± σ):      5.997 s ±  0.066 s    [User: 7.707 s, System: 1.831 s]
  Range (min … max):    5.870 s …  6.072 s    10 runs
 
Benchmark 2: node ./after.cjs workspaces foreach run
  Time (mean ± σ):      5.887 s ±  0.072 s    [User: 7.533 s, System: 1.683 s]
  Range (min … max):    5.769 s …  5.977 s    10 runs
 
Summary
  'node ./after.cjs workspaces foreach run' ran
    1.02 ± 0.02 times faster than 'node ./before.cjs workspaces foreach run'

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.