fix(core): handle case where package.json is not available at CWD

[Josh-Cena]

Motivation

Fix #7161. Moreover, this PR adds a DOCUSAURUS_VERSION env so that different modules can read this version without re-importing the package.json file.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Delete website/package.json, and add "dev": "cd website && docusaurus start", to the root package.json. The site starts gracefully:

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7187--docusaurus-2.netlify.app/

[github-actions]

Size Change: +117 B (0%)

Total Size: 799 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	107 kB	+232 B (0%)
website/build/assets/js/main.********.js	604 kB	-115 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50 kB
website/build/index.html	38.8 kB

_{compressed-size-action}

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[Josh-Cena]

We could have used pkg-up or similar utilities, but I'm not sure if that would lead to unexpected results, like finding a package.json that's below the desired one... If we are to make docusaurus upgrade (#4082), the package.json file has to be unambiguous, so it's better if we can set a standard practice from the beginning.

[slorber]

🤷‍♂️ not really fan, particularly the DOCUSAURUS_VERSION env var