There are other possible annotations in the build file.

You can get an idea of what it looks like by reading the Bazel docs: https://bazel.build/versions/master/docs/be/overview.html

Storing a few text files at Google doesn't cost millions of dollars, BTW.

They don't use git or any other distributed version control system, so there is no incentive to keep it small. And anything outside the source control system isn't accessible to all the tools that use it, so it would introduce complexity.

Then you have to run a tool that processes .go source files in order to perform dependency analysis. Consistency is a virtue.

Nah, because it would cost the company billions of dollars in lost productivity waiting for these files to get re-processed every time someone built the thing. Google's general philosophy is that humans are expensive and computers are cheap, so pretty much anything that helps the humans go faster is a going to be a net benefit in the long run.

People rarely get promoted for saving money.