Try to find the addons dir / namespace package

    Returns addons_dir, addons_ns

def _find_addons_dir():
    """ Try to find the addons dir / namespace package

    Returns addons_dir, addons_ns
    """
    res = set()
    for odoo_version_info in ODOO_VERSION_INFO.values():
        addons_ns = odoo_version_info['addons_ns']
        addons_dir = os.path.join(*addons_ns.split('.'))
        if os.path.isdir(addons_dir):
            if not odoo_version_info['namespace_packages'] or \
                    os.path.isfile(os.path.join(addons_dir, '__init__.py')):
                res.add((addons_dir, addons_ns))
    if len(res) == 0:
        raise RuntimeError("No addons namespace found.")
    if len(res) > 1:
        raise RuntimeError("More than one addons namespace found.")
    return res.pop()