build(aio): refactor dgeni packages

This is to tidy up the `author-packagse`, which currently duplicates a
lot of the configuration in the main packages. We need to
DRY this up so that we don't fall foul of a change in one being missed in
the other.

aio/tools/transforms/angular-base-package/processors/fixInternalDocumentLinks.js

@@ -0,0 +1,24 @@
+/**
+ * @dgProcessor fixInternalDocumentLinks
+ * @description
+ * Add in the document path to links that start with a hash.
+ * This is important when the web app has a base href in place,
+ * since links like: `<a href="#some-id">` would get mapped to
+ * the URL `base/#some-id` even if the current location is `base/some/doc`.
+ */
+module.exports = function fixInternalDocumentLinks() {
+
+  var INTERNAL_LINK = /(<a [^>]*href=")(#[^"]*)/g;
+
+  return {
+    $runAfter: ['inlineTagProcessor'],
+    $runBefore: ['convertToJsonProcessor'],
+    $process: function(docs) {
+      docs.forEach(doc => {
+        doc.renderedContent = doc.renderedContent.replace(INTERNAL_LINK, (_, pre, hash) => {
+          return pre + doc.path + hash;
+        });
+      });
+    }
+  };
+};