Pegasus frontend: Handle 'extra' based on command/launch lines

[nicjansma]

This changes Pegasus output to handle the -e <STRING> argument and command:/launch: differently:

• If -e was not provided (or was provided empty), utilize the existing command: OR launch: tag (which are aliases of each other, and the later is what the Metadata Generator writes)
  • Otherwise, there was no way to tell skyscraper to just use the existing pre-fab'd metadata from e.g. the Metadata Generator
• If -e was provided, always use that
• Always write command (or launch) last, since that is generally a multi-line command