[gazelle] Support -index=lazy

Gazelle supports `-index=lazy` to speed up indexing directories. This currently fails on Python with a bunch of errors like
```
my_project/foo/bar.py: "foo.other.baz" is an invalid dependency
```

Perhaps a `gazelle:py_search` directive is needed?

使用 Python 和 `-index=lazy` 在 Gazelle 中重现该失败，重点关注报告的无效依赖错误，例如 `foo.other.baz`。调查是否需要 `gazelle:py_search` 指令；当 Python 的延迟索引在没有这些依赖错误的情况下正常工作时，即视为完成。

由索引模型根据 Issue 内容生成。