allow automatic upload of CM information with packages

@mit-mit @isoos

Dartdoc now supports linking directly to source code for generated API documentation. However, in order for this to work with the pub site, users have to [autogenerate dartdoc_options.yaml files themselves](https://github.com/dart-lang/dartdoc/blob/9eadb603220ca6647e2f2183e67f5ae2c0e0d66a/tool/grind.dart#L786) with the necessary revision and linking information, and upload them with the package.

For a common case, uploading a package to pub from a github-backed repo for a package, it seems there should be some way to make this easier. It will probably require a design that spans dartdoc, pub, and the pub site.