This copies a flake and all its inputs recursively to a store (e.g. a
binary cache). This is intended to enable long-term reproducibility
for flakes. However this will also require #3253.

Example:

  $ nix flake archive --json --to file:///tmp/my-cache nixops
  {"path":"/nix/store/272igzkgl1gdzmabsjvb2kb2zqbphb3p-source","inputs":{"nixops-aws":{"path":"/nix/store/ybcykw13gr7iq1pzg18iyibbcv8k9q1v-source","inputs":{}},"nixops-hetzner":{"path":"/nix/store/6yn0205x3nz55w8ms3335p2841javz2d-source","inputs":{}},"nixpkgs":{"path":"/nix/store/li3lkr2ajrzphqqz3jj2avndnyd3i5lc-source","inputs":{}}}}

  $ ll /tmp/my-cache
  total 16
  -rw-r--r-- 1 eelco users 403 Jan 30 01:01 272igzkgl1gdzmabsjvb2kb2zqbphb3p.narinfo
  -rw-r--r-- 1 eelco users 403 Jan 30 01:01 6yn0205x3nz55w8ms3335p2841javz2d.narinfo
  -rw-r--r-- 1 eelco users 408 Jan 30 01:01 li3lkr2ajrzphqqz3jj2avndnyd3i5lc.narinfo
  drwxr-xr-x 2 eelco users   6 Jan 30 01:01 nar
  -rw-r--r-- 1 eelco users  21 Jan 30 01:01 nix-cache-info
  -rw-r--r-- 1 eelco users 404 Jan 30 01:01 ybcykw13gr7iq1pzg18iyibbcv8k9q1v.narinfo

Fixes #3336.