fix(v2): i18n perf issue: getTranslationFile() should not load content again

[slorber]

Motivation

i18n perf: the docs plugin read twice the markdown files, because it tries to load content again when trying to retrieve translation files to use.

Before, reloading the content that has already been loaded:

    async getTranslationFiles() {
      return getLoadedContentTranslationFiles(await this.loadContent());
    },

After: reusing the content that has already been loaded:

    async getTranslationFiles({content}) {
      return getLoadedContentTranslationFiles(content);
    },

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

Not sure how. It's hard to prevent a plugin author to call this.loadContent() anywhere and have a perf issue, including maintainers in core plugins. It's not performant but it works.

[netlify]

[V1]

Built with commit 4b27a03

https://deploy-preview-4593--docusaurus-1.netlify.app

[netlify]

[V2]

Built with commit 4b27a03

https://deploy-preview-4593--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	63
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4593--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 576 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.2 kB	0 B
website/build/assets/js/main.********.js	401 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	61.1 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	26.1 kB	0 B

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