[build2] fmt library module

[Klaim - Joël Lamotte]

Correction:  I gathered my notes from that time and indeed the fmt module
support for build2 is not complete yet (in addition to the issues pointed
out by Boris).
I attempted to fix the obvious issues, the commits are in the `modules`
branch of the package repository.

However, the reason I didn't make it work yet or publish it:
 - gcc11 will ICE when trying to build this module (that's also what the
fmt devs told me would happen, so no surprises);
 - msvc modules support is not possible to enable with build2 v0.14.0 (the
devs said it would work otherwise);
 - clang does not really support standard modules yet;
So at the moment I have no way to properly test this. That's where I
stopped before, if my notes are correct.
I was assured by the devs that the module does work with msvc (not gcc) and
I confirm it works when using msbuild.
I will attempt to publish the fixes once build2 re-enables msvc modules.

Anyway as Boris said, you better wait for more/better support for modules
by the various implementations before starting to use this module.

Joël
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