def parse_requirements_fname(dep_name):
    """Parse requirements file path from dependency declaration (-r<filepath>).

    >>> parse_requirements_fname('pep8')
    >>> parse_requirements_fname('-rrequirements.txt')
    'requirements.txt'

    :param dep_name: Name of the dependency
    :return: Requirements file path specified in the dependency declaration
        if specified otherwise None
    :rtype: str or None
    """
    req_option = '-r'
    if dep_name.startswith(req_option):
        return dep_name[len(req_option):]
