fix(jest-haste-map): Make watchman existence check lazy+async

[robhogan]

Summary

Currently, jest-haste-map checks for watchman existence at import time (top level) with a blocking execSync call. This has a few downsides

• Blocking at the top level holds up main thread (by 20-200ms) at a time when there's CPU work to be done.
• execSync spawns a shell, which isn't necessary here - execFile is more efficient.
• The exec happens totally unnecessarily even if the consumer specifies useWatchman: false.

This PR extracts this to an async utility function, executed lazily and memoized by the core class.

Test plan

• New unit tests cover lib/isWatchmanInstalled.ts and lazy execution

• Behaviour of execFile when watchman is installed:

$ node -e "util.promisify(child_process.execFile)('watchman', ['--version']).then(console.log)"
{ stdout: '20220411\n', stderr: '' }

• And when we try to exec something unavailable:

$ node -e "util.promisify(child_process.execFile)('nonexistentthing', ['--version']).then(console.log)"

node:internal/errors:464
   ErrorCaptureStackTrace(err);
   ^

Error: spawn nonexistentthing ENOENT
   at Process.ChildProcess._handle.onexit (node:internal/child_process:282:19)
   at onErrorNT (node:internal/child_process:477:16)
   at processTicksAndRejections (node:internal/process/task_queues:83:21) {
 errno: -2,
 code: 'ENOENT',
 syscall: 'spawn nonexistentthing',
 path: 'nonexistentthing',
 spawnargs: [ '--version' ],
 cmd: 'nonexistentthing --version',
 stdout: '',
 stderr: ''
}

[SimenB]

Ooh, great stuff! 👍

[github-actions]

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