log the command about to be run

[joachim-n]

See drush-ops/drush#4151

It would be useful if Drush's debug output said the command class and method that was about to be run.

CommandProcessor has LoggerAwareTrait, so it can log, but Drush uses constants from Drush\Log\LogLevel, so I don't know how we do that over here.

[weitzman]

No need for constants. Just use methods that correspond like $logger->warning(), $logger->info().