Solve the problem that <Flutter/Flutter.h> cannot be imported when a pod transitive depends on Flutter (#125610)

![image](https://user-images.githubusercontent.com/8318578/234780282-89b18d27-df49-4b4e-88b5-c9d17cf3334f.png)
![image](https://user-images.githubusercontent.com/8318578/234780668-901ab816-5b6b-4d87-a6f4-120b5852580c.png)
If a pod transitive depends on a pod containing a framework, cocoapods will add the path of the framework to its FRAMEWORK_SEARCH_PATHS.
So I modified the relevant logic in podhelper, hoping to be consistent with the behavior of cocoapods.

Fixes https://github.com/flutter/flutter/issues/126251.