refactor(mdx-loader): read metadata from memory (loaded content) instead of fs (#10457)

* mdx loader shouldn't read metadata from file system but from memory

* comments

* refactor: apply lint autofix

* apply same for blog

* apply same for blog

* refactor: apply lint autofix

* apply same for pages

packages/docusaurus-plugin-content-docs/src/contentHelpers.ts

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import type {DocMetadata, LoadedContent} from '@docusaurus/plugin-content-docs';
+
+function indexDocsBySource(content: LoadedContent): Map<string, DocMetadata> {
+  const allDocs = content.loadedVersions.flatMap((v) => v.docs);
+  return new Map(allDocs.map((doc) => [doc.source, doc]));
+}
+
+// TODO this is bad, we should have a better way to do this (new lifecycle?)
+//  The source to doc/permalink is a mutable map passed to the mdx loader
+//  See https://github.com/facebook/docusaurus/pull/10457
+//  See https://github.com/facebook/docusaurus/pull/10185
+export function createContentHelpers() {
+  const sourceToDoc = new Map<string, DocMetadata>();
+  const sourceToPermalink = new Map<string, string>();
+
+  // Mutable map update :/
+  function updateContent(content: LoadedContent): void {
+    sourceToDoc.clear();
+    sourceToPermalink.clear();
+    indexDocsBySource(content).forEach((value, key) => {
+      sourceToDoc.set(key, value);
+      sourceToPermalink.set(key, value.permalink);
+    });
+  }
+
+  return {updateContent, sourceToDoc, sourceToPermalink};
+}