In particular, this now works:

  $ nix path-info --eval-store auto --store https://cache.nixos.org nixpkgs#hello

Previously this would fail as it would try to upload the hello .drv to
cache.nixos.org. Now the .drv is instantiated in the local store, and
then we check for the existence of the outputs in cache.nixos.org.

With this, we don't have to copy the entire .drv closure to the
destination store ahead of time (or at all). Instead, buildPaths()
reads .drv files from the eval store and copies inputSrcs to the
destination store if it needs to build a derivation.

Issue NixOS#5025.

This adds a new store operation 'addMultipleToStore' that reads a
number of NARs and ValidPathInfos from a Source, allowing any number
of store paths to be copied in a single call. This is much faster on
high-latency links when copying a lot of small files, like .drv
closures.

For example, on a connection with an 50 ms delay:

Before:

  $ nix copy --to 'unix:///tmp/proxy-socket?root=/tmp/dest-chroot' \
    /nix/store/90jjw94xiyg5drj70whm9yll6xjj0ca9-hello-2.10.drv \
    --derivation --no-check-sigs
  real    0m57.868s
  user    0m0.103s
  sys     0m0.056s

After:

  real    0m0.690s
  user    0m0.017s
  sys     0m0.011s