package updater:
  try fetching tarballs;
  cache results, including negative;
  add an option to ignore the negative cache.
update to unreleased nix
package-updater: don't mangle urls if package version is not known
Store the location of packages in nixpkgs tree
put nixpkgs checkout location into a property so that things can be more flexible in the
future
DistroPackage::Nix: put package metadata parsing code into a separate method to be used later
by patch generator to verify patches
DistroPackage::Nix: allow parsing of a single nix package so that patches can be verified to
apply cleanly
package updater: generate patches, checking that the patches cause the expected modifications to packages' version, source url and hash
web interface: expose patches, show patch links alongside new versions.
Make updater and web interface more resilient to transient inconsistencies in the database
web interface: allow the database to be downloaded
DistroPackage::Nix: greatly speed up fetching of metadata for a single package.