Add new args to create_data_tar.py

[keerthi-go]

refactor(create_data_tar): CLI flag changes and output path update. This is needed to cleanly seperate build artifacts of multiple packages having same parent folder.

• Rename --output-dir to --path-to-changes (accepts .changes file or dir; newest .changes chosen if dir)
• Remove --changes-file
• Add --output-tar; tar now written to /prebuilt_/ (or fallback to work_dir when not set)
• Update find_changes_file to accept path/file, adjust help/docstring accordingly