Sets `builder` configuration, and returns all the builder steps
    necessary to build `spec` and its dependencies.

    Traverses the dependencies in depth-first order, honoring the sequencing
    in each 'depends_on' list.

def steps_for_spec(builder, spec, processed_modules=None):
    '''
    Sets `builder` configuration, and returns all the builder steps
    necessary to build `spec` and its dependencies.

    Traverses the dependencies in depth-first order, honoring the sequencing
    in each 'depends_on' list.
    '''
    if processed_modules is None:
        processed_modules = set()
    steps = []
    for module in spec.get('depends_on', []):
        if module not in processed_modules:
            processed_modules.add(module)
            steps.extend(steps_for_spec(
                builder,
                module.fbcode_builder_spec(builder),
                processed_modules
            ))
    steps.extend(spec.get('steps', []))
    return steps