Make CheckAWSAuth, UploadToS3, ImportSnapshot, and RegisterAMI public so a build server can run them individually (e.g. Marketplace publishing after RegisterAMI). Rename BuildAMI to BuildAndImportAMI, now a thin orchestrator over them. Each step records into Result and returns its artifact; the AWS steps guard on their predecessor and error clearly if called out of order. Updates #1866 Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>