don't mark `import a as a` as unused import

[jorenham]

Describe the Bug

pyrefly reports an unused-import warning for the following implicit re-export:

from math import tau as tau

The other type-checkers don't, because the convention (it might even be a typing spec thing) is to treat import {name} as {name} as a re-export, i.e. {name} should be a public symbol in that module.

See https://github.com/scipy/scipy-stubs/blob/4b2ee9a7a917ebd8918caf1ab51146ba4898a34b/scipy-stubs/_lib/_array_api_compat_vendor.pyi for a real-world example of this (at runtime there is no __all__).

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAENqALgBZ0Q3G6VN1OoBXOqjh9BAHXQgAvgBoQqAMZMIANxgAxaDAposeImRlA

(Only applicable for extension issues) IDE Information

No response

[abelcheung]

The other type-checkers don't, because the convention (it might even be a typing spec thing) is to treat import {name} as {name} as a re-export, i.e. {name} should be a public symbol in that module.

It is indeed written in typing spec.

[AryanBagade]

working on it!

[AryanBagade]

done!