[npm-check-fork] Replace package-json and throat dependencies with local implementations

[TheLarkInn]

Summary

The @rushstack/npm-check-fork package depends on package-json and throat, which should be replaced with existing Rush Stack utilities:

• package-json: ESM-only module with a heavy dependency tree (~500KB). We only use it to fetch basic registry metadata.
• throat: Concurrency limiter that duplicates functionality already in @rushstack/node-core-library

Details

Proposed solution:

1. Replace package-json with a minimal NpmRegistryClient class using the existing WebClient from @rushstack/rush-lib
2. Replace throat with Async.forEachAsync from @rushstack/node-core-library

Benefits:

• Eliminates ESM compatibility issues with CJS consumers
• Reduces bundle size and dependency footprint
• Integrates with Rush's existing proxy configuration
• Uses utilities already in the codebase
• Reduces chances of CVE's from upstream dependencies

Standard questions

Question	Answer
Package name:	@rushstack/npm-check-fork
Package version?	5.0.x
Operating system?	All
Would you consider contributing a PR?	Yes
Node.js version (node -v)?	18.x+