Convert ``relative`` sls reference into absolute, relative to ``sls``.

def _relative_to_abs_sls(relative, sls):
    '''
    Convert ``relative`` sls reference into absolute, relative to ``sls``.
    '''
    levels, suffix = re.match(r'^(\.+)(.*)$', relative).groups()
    level_count = len(levels)
    p_comps = sls.split('.')
    if level_count > len(p_comps):
        raise SaltRenderError(
            'Attempted relative include goes beyond top level package'
        )
    return '.'.join(p_comps[:-level_count] + [suffix])