[build2] GCC modulemap informations

[Arthur Laurent]

Hi there, i'm a contributor of XMake build system C++20 module support
we have an implementation of C++20 module mapper with file based API, 
but i find this API poorly designed and tricky to use
so i want to try the server based API
i see you have implemented this, but i don't understand how it work 
exactly inside your build process can i have some precision on how it is 
implemented if possible ? do you use sockets or pipes ? the module 
mapper server is a thread of build2 ? an external process ? and is it 
really better than file based API ?