Reduce differences in user code from Bazel

Discussed with @ndmitchell last week, that some Bazel users may want to try Buck2 without having to make serious changes to their project, for example use the same `BUILD` files. Even better, any serious Bazel user who wants to switch will need their repo to build&test with both tools during a migration window.

A good first experiment would be to take a small Bazel project and add minimal files to make it work with both.