Use C++ modules

[L-as]

Perhaps a duplicate issue.

C++ modules seem objectively better than headers.

Perhaps some of the stuff that Nix does with headers can't be
replicated with modules but it seems like you can use them both at the same time.

For reference: https://clang.llvm.org/docs/StandardCPlusPlusModules.html

[L-as]

Might be easier with #3160

[L-as]

See https://github.com/mesonbuild/meson/blob/864919973f73ec2c6ae6ca01be7fe5e8b950df12/test%20cases/unit/85%20cpp%20modules/gcc/meson.build for example with Meson, seems very straight-forward