[tools] Update info output and add validate mode.

- Handle multeple assets, licenses, and listings.
- Making an attempt to improve info output.
  - More output description.
  - Indentation with '=' character.
  - Blank line between outputs.
- Add validate mode.
  - Use --validate to enable. Not enabled by default since it is likely
    to cause confusion in the common case of not having all data on the
    same page. A future tool will do more complete validation.
  - Keeps track of seen assets, licenses, and listings.
  - If all hashes are present, data is valid.
  - If hashes for same id mismatch, data is unlikely to be valid.
  - If hashes all hashes are not present, it is not known if data is
    valid, which may be ok.