Add support for implied task names on the command line

[geoffxy]

If a task has the same name as their parent directory, it would be convenient if we could refer to it on the command line (when running cond run) without its trailing name (i.e., just the task identifier path).

For example task world defined in hello/world/COND could also be referred to as //hello/world as well instead of its fully qualified name, //hello/world:world. This would only apply to the CLI, not to dependencies in COND files.