C++ module implementation is illegal

[avikivity]

Compiling module support with modern clang fails. Clang complains that module cannot be in a #ifdef.

We should have an extra file that imports the headers as non-modules and re-exports them as modules, following libstdc++ [1].

[1] https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B23/std.cc.in

[avikivity]

Also found this syntax:

export import <seastar/core/reactor.hh>

[avikivity]

Hmm, that doesn't work will with -DNAME=value, which we do use.

[avikivity]

Instructions for copilot:

1. remove the module stuff in .cc files (logically a revert of aa1c8b3)
2. add a new seastar-module.cppm in the spirit of https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B23/std.cc.in
3. adjust the cmake build scripts
4. test that it works