Make packlist validate more / make validate-versions follow links

[black-sliver]

• Currently, versions.json could point to a dead link and it won't be caught since we only validate the schema.
  • There could be a command line switch in validate-versions.py to check latest or all entries.
  • We would want to download the zip, compare the SHA, validate it's a ZIP and maybe check the manifest?
• Currently only packlist -> versions is checked, not packlist -> versions -> pack.zip, so a mismatch in UUID will not be caught.
  • The checks to do should be the same as above + the UUID.
  • There could be a command line switch in validate-packs.py to follow the latest or all entries of individual version.json files.
  • We may want to cache downloads to avoid hitting rate limits?

#364 shows that it's easy to get wrong and that's not the first to get it wrong and I also almost missed it.