Comment 1 for bug 1659508

The problem was introduced with my "commands:cmd_session: don't get_exporter_names() when not needed" 84e3b2c96566df7322a2dd661c260b60604b55bc.

For each subcommand that we have in plainbox we have split them to (at least) two files. cmd_* and inv_*. The former having the declaration of arg and its options and the latter having the implementation. This ensures that every module that invocation needs is only loaded when we invoke the command.

With my 84e3b2 patch I removed checking of all available exporters in the cmd_ phase. This led to a plainbox starting up 4x faster. I left '?' as the only possible value of --output-format, and when invoked with that arg, it would actually pull all exporters from providers and list them, as it would run through the inv_ phase.