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-api-package/processors/markBarredODocsAsPrivate.js

@@ -0,0 +1,13 @@
+module.exports = function markBarredODocsAsPrivate() {
+  return {
+    $runAfter: ['readTypeScriptModules'],
+    $runBefore: ['adding-extra-docs'],
+    $process: function(docs) {
+      docs.forEach(doc => {
+        if (doc.name && doc.name.indexOf('ɵ') === 0) {
+          doc.privateExport = true;
+        }
+      });
+    }
+  };
+};