通过amis updateProps()更新的schema，不支持definitions

复现：
1.
通过amis.embed()加载schema1.json，里面配置一个definitions。
然后通过 updateProps()更新为schema2.json，调用schema1.json中的definitions**成功**。

2.
通过amis.embed()加载schema1.json。
然后通过 updateProps()更新为schema2.json，并配置definitions，然后调用它**失败**。提示 找不到对应的渲染器。Path: page/body/..../undefined
{
"$ref": "tab"
}

Reproduce the issue with amis.embed(), two schemas, updateProps(), definitions, and a $ref to tab. Trace how the updated schema resolves definitions and renderer lookup, then verify that definitions supplied by schema2 resolve successfully without the undefined-renderer error.

Written by the indexing model from the issue text.