[BUG] `cpp2util.h` should use `__cpp_lib_modules`

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**[T](#title)itle**: `cpp2util.h` should use `__cpp_lib_modules`.

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**[D](#desc)escription**:

`cpp2util.h` uses `__cpp_modules` to decide whether to `import std;`.
But it should be using [`__cpp_lib_modules`](https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#__cpp_lib_modules).
