Update package management

This project was started when Python package managers were really proliferating. It's currently focused on the tool the user has chosen for package management, rather than the specification file.

• Switch from trying to detect the package manager used, to the specification file that's found.
  • requirements.txt/ requirements.in
  • pyproject.toml
  • Pipfile
  • others?
• Address @ git+ dependencies. An updated dependency management approach should address this.
  • See dsd-flyio issue 28.