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/convertPrivateClassesToInterfaces.js

@@ -0,0 +1,11 @@
+module.exports = function convertPrivateClassesToInterfacesProcessor(
+    convertPrivateClassesToInterfaces) {
+  return {
+    $runAfter: ['processing-docs'],
+    $runBefore: ['docs-processed'],
+    $process: function(docs) {
+      convertPrivateClassesToInterfaces(docs, false);
+      return docs;
+    }
+  };
+};