Find bobfile in parent directory

[rdnt]

bob should additionally search for a bobfile in parent directories. This would allow bob commands to be run without going back to the project root.

[zombiemaker]

How about providing a reference to the root of the Bob workspace similar to how Bazel provides "//" to refer to the main Bazel repository root directory. If bob.yaml files are used throughout the repository and they need to reference common resources (e.g., Nix expression files) that are located at a specific directory under the workspace root directory, relative paths have to be used which is a not-nice way to access resources.