Per discussion, pulled out a lot of the Bazel mentions out separately and trying to avoid making design choices for it here. (trying to just use it for illustrative purposes)

Broadly LG, but there are a few areas where I think we'd benefit from being clearer about what is proposed versus what is being left for us to work out as part of the implementation.

Thanks, LGTM.

I'd like @chandlerc to have a look with his painter hat on, specifically to check the defaults match his expectations (default.carbon as the default library in a package, and a default output binary named program), but I'm happy.