Integrate PackageManager with the new package distro

[fingolfin]

The new package distribution at https://github.com/gap-system/PackageDistro collects meta data for all packages distributed with GAP.

One could teach PackageManager to use that data directly instead of going through PackageInfoURLs. This way, a single operation could get all the metadata (e.g. by fetching https://github.com/gap-system/PackageDistro/releases/download/latest/package-infos.json.gz) and parse it. By using this, and possible also tags like v4.11.1 (perhaps eventually also branches like stable-4.11, see gap-system/PackageDistro#297), one could do a variety of things:

• let the user choose between updating to the latest...
  • released version;
  • accepted version;
  • shipped version
• implementing a CheckForUpdates successor (see Replacement for CheckForUpdates #46) should be fairly easy
• implementing UpdateAllPackages (see UpdateAllPackages #45) similarly should be straight forward