Refactorings and fixups

Streamline the code and fix a couple of issues.

• 2c7ed0d1 installability: Avoid positional arguments, use flags insteads
• 939a5127 installability: Print a summary on stdout
• 14b80de2 installability: Refactor check_packages()
• 4150dde5 installability: Get rid of a global variable
• f1829a9a installability: Refactor Packages file download to be more reusable
• 87597b3e installability: Rework downloader to avoid external commands
• e9ae18bd installability: Do not hardcode the Packages.gz components
• a9438781 installability: Make --release mandatory
• 78c63966 installability: Add --fg and --bg to limit execution
• 5e63a066 installability: Include successful packages in the JUnit output
• c7e2b02a installability: Do not whitelist packages that are not broken
• c42289e2 installability: Include explanations for failures
• daf9f8c6 installability: Simplify internal structures