Make library work with C++20 modules

[nlohmann]

• Added a test to compile the library as module with the latest Clang and GCC.
• Used the fix from Compiler error exposes TU-local entity on gcc-trunk while compiling the library as a module #4759.

Closes #4759.

[coveralls]

coverage: 99.188%. remained the same
when pulling 5dfd121 on issue4759-module
into 51a77f1 on develop.

[slavek-kucera]

LGTM

[ChuanqiXu9]

Given you already make this, would you like to offer module interfaces for this library officially?

[nlohmann]

What would make it "official"?

[ChuanqiXu9]

What would make it "official"?

If users can import json; (or import nlohmann.json;) after they installed the repo. Concretely. I think it is fine to move tests/module_cpp20/json.cpp (I feel better to rename it to json.cppm) in this patch to the source directory and add an cmake option NLOHMANN_JSON_MODULES, and when this option is ON, the module interface unit file will be installed to the target path.

In another word, if this project offers the module interface file, I think the project offers modules officially.

[nlohmann]

Thanks for the clarification. I am hesitant here, because currently we lack MSVC support. And before offering an official module, I would need to make sure that the whole test suite runs then, too. So I think it requires some more effort, but I think we're on a good track.

[slavek-kucera]

Both MSVC and gcc still struggle with modules. I opened a case for MSVC in April. And there is a case for gcc opened back in 2021.