Merge pull request #1954 from reitermarkus/refactor-artifacts

Library/Homebrew/cask/lib/hbc/artifact/base.rb

Library/Homebrew/cask/lib/hbc/artifact/installer.rb

Library/Homebrew/cask/lib/hbc/artifact/nested_container.rb

Library/Homebrew/cask/lib/hbc/artifact/pkg.rb

Library/Homebrew/cask/lib/hbc/artifact/stage_only.rb

Library/Homebrew/cask/lib/hbc/artifact/uninstall_base.rb

Library/Homebrew/cask/lib/hbc/artifact/zap.rb

Library/Homebrew/cask/lib/hbc/installer.rb

Library/Homebrew/cask/test/cask/artifact/pkg_test.rb

Library/Homebrew/cask/test/cask/artifact/uninstall_test.rb

Library/Homebrew/cask/test/cask/artifact/zap_test.rb