Use fmt::format for loggers

[ken-matsui]

https://github.com/poac-dev/poac/blob/23471b6c352809dff0d0655cbb47586e1c8e3358/src/Logger.hpp

For error, warn, info, debug, and trace, we'd like to use fmt::format style formatting instead of std::ostream (internal implementation details). For example, the following usage is not that clear:

https://github.com/poac-dev/poac/blob/23471b6c352809dff0d0655cbb47586e1c8e3358/src/main.cc#L94-L96

We may want to do like this instead:

logger::error(
    "'poac {}' failed with exit code `{}`", *itr, exitCode
 ); 

We can use fmt::format for arguments to these loggers and we do, but the new interface will be clearer:

https://github.com/poac-dev/poac/blob/23471b6c352809dff0d0655cbb47586e1c8e3358/src/Cmd/Build.cc#L52-L58

logger::info can be:

logger::info( 
    "Compiling", "{} v{} ({})", packageName,
    getPackageVersion().toString(),
    getProjectBasePath().string()
);

Theoretically, error, warn, debug, and trace's function arguments would be like:

(fmt::format_string<T...> fmt, T&&... args)

And info has:

(std::string_view header, fmt::format_string<T...> fmt, T&&... args)

Note that the actual implementation may be different as to conform to the current implementation style.