Improve testing & output / error handling

[rgoldberg]

Improve testing & output / error handling. Will possibly separate into multiple issues later.

• Ensure everything can be mocked to be tested
  • Refactor Downloader.swift as a protocol so it can be mocked to test purchase, install, lucky, and upgrade #591
• Use proper dependency injection
• Possibly change output generation & error handling. Either:
  • capture all printed output via captureStream
  • allow print target to be switched from stdout to a mutable String / redirect stdout to a String
  • collect multiple good outputs, multiple nonfatal warnings, and multiple fatal errors in one mechanism to handle all information (like Result, but allowing multiple values for success, nonfatal errors, and fatal errors)
• Possibly test by running from command line instead of calling Swift code
• Properly apportion code between Commands & Controllers
• Possibly migrate to swift-testing (requires macOS 10.15+)
• Replace XCTest with either Quick/Nimble or swift-testing