Initial generator for google API packages

Holds a configuration file with:

* which API goes into which package
* which API to ignore

Then just calls the discovery generator to do the job.

The configuration needs to be updated with more information (version,
description, etc.) which goes into the pubspec.yaml

R=kustermann@google.com
BUG=

Review URL: https://codereview.chromium.org//475413002