[build2] fmt library module

[Lucas Ransan]

Hello,

I'm trying to use the {fmt} library with C++20 modules, without
success. I added

    depends: fmt ~8.1.0

to ./manifest,

    cxx.std = experimental
    cxx.features.modules = true

to ./build/root.build,

    import fmt;

to ./src/main.cxx, but build2 says it is unable to resolve module fmt.

I can see in b -sV output

    config fmt@/gcc/fmt-8.1.1/
      enable_modules false

which is weird, considering config.fmt.enable_modules is defined as
equal to $cxx.features.modules (which is true).

Is this a bug? Did I miss something?

Lucas