Produce a mapping of identifier: requirement from the section.

def _get_requirements(model, section_name):
    """Produce a mapping of identifier: requirement from the section.
    """
    if not model:
        return {}
    return {identify_requirment(r): r for r in (
        requirementslib.Requirement.from_pipfile(name, package._data)
        for name, package in model.get(section_name, {}).items()
    )}