feat(v2): plugins injectHtmlTags + configureWebpack should receive content loaded

[slorber]

Motivation

Plugin lifecycles that are triggered after we loaded the plugin content should permit to access that plugin content.

The following should work:

module.exports = function (context, options) {
  return {
    name: 'docusaurus-plugin',
    loadContent: async () => {
      return {remoteHeadTags: await fetchHeadTagsFromAPI()};
    },
    injectHtmlTags({content}) {
      return {
        headTags: content.remoteHeadTags,       
      };
    },
  };
};

Also injecting the content in the configureWebpack lifecycle, as we compute the mdx/remark config by using the loaded content for resolving relative file path links to URLs. This permits to avoid an unelegant side-effect we previously had in loadContent:

        // TODO annoying side effect!
        Object.values(docs).forEach((loadedDoc) => {
          const {source, permalink} = loadedDoc;
          sourceToPermalink[source] = permalink;
        });

Mentioned in #4923 (comment)

Test Plan

test + dogfood in internal plugins

[slorber]

cc @jsamr is this what you had in mind?

[jsamr]

@slorber Absolutely 👌 Thank you!

[netlify]

✔️ [V2]

🔨 Explore the source changes: 548fa95

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60d1fe30213eff00079caa43

😎 Browse the preview: https://deploy-preview-5037--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	64
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5037--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 569 kB

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	87.8 kB
website/build/assets/js/main.********.js	385 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	65.8 kB
website/build/docs/introduction/index.html	235 B
website/build/index.html	30.5 kB

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