Move functionality from Artifact class to ArtifactCommand class

[AlexSkrypnyk]

#42 allows us to further improve the architecture of this package.

BLOCKED by #64

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We now need to reconcile Artifact and ArtifactCommand classes.

This ticket is about moving the functionality of the Artifact to ArtifactCommand.

Please note that there is be a follow-up ticket to replace the logger with monolog.

Solution direction.

1. Merge the functionality of Artifact to ArtifactCommand. The Artifact::artifact() should become ArtifactCommand::processArtifact() and be called from ArtifactCommand::execute().

2. Update `ArtifactCommand::execute() to correctly return exit codes on exception.
   The code should look like:

  protected function execute(InputInterface $input, OutputInterface $output): int {
    $this->logger = self::logInit($output);

    $this->checkRequirements();

    try {
      $this->processArtifact();
    }
    catch (\Exception $e) {
      $output->writeln('<error>' . $e->getMessage() . '</error>');

      return Command::FAILURE;
    }

    return Command::SUCCESS;
  }