chore(v2): make v2 docs plugin legacy (#1639)

* chore(v2): make v2 docs plugin legacy

* chore(v2): update fixtures

packages/docusaurus-plugin-content-docs-legacy/src/markdown/index.js

@@ -0,0 +1,60 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+const {getOptions} = require('loader-utils');
+const {resolve} = require('url');
+
+module.exports = async function(fileString) {
+  const callback = this.async();
+  const options = Object.assign({}, getOptions(this), {
+    filepath: this.resourcePath,
+  });
+  const {docsDir, sourceToPermalink} = options;
+
+  // Determine the source dir. e.g: /docs, /website/versioned_docs/version-1.0.0
+  let sourceDir;
+  const thisSource = this.resourcePath;
+  if (thisSource.startsWith(docsDir)) {
+    sourceDir = docsDir;
+  }
+
+  let content = fileString;
+
+  // Replace internal markdown linking (except in fenced blocks).
+  if (sourceDir) {
+    let fencedBlock = false;
+    const lines = content.split('\n').map(line => {
+      if (line.trim().startsWith('```')) {
+        fencedBlock = !fencedBlock;
+      }
+      if (fencedBlock) return line;
+
+      let modifiedLine = line;
+      // Replace inline-style links or reference-style links e.g:
+      // This is [Document 1](doc1.md) -> we replace this doc1.md with correct link
+      // [doc1]: doc1.md -> we replace this doc1.md with correct link
+      const mdRegex = /(?:(?:\]\()|(?:\]:\s?))(?!https)([^'")\]\s>]+\.md)/g;
+      let mdMatch = mdRegex.exec(modifiedLine);
+      while (mdMatch !== null) {
+        // Replace it to correct html link.
+        const mdLink = mdMatch[1];
+        const targetSource = `${sourceDir}/${mdLink}`;
+        const permalink =
+          sourceToPermalink[resolve(thisSource, mdLink)] ||
+          sourceToPermalink[targetSource];
+        if (permalink) {
+          modifiedLine = modifiedLine.replace(mdLink, permalink);
+        }
+        mdMatch = mdRegex.exec(modifiedLine);
+      }
+      return modifiedLine;
+    });
+    content = lines.join('\n');
+  }
+
+  return callback(null, content);
+};