handle stalled/interrupted builds

[thibautjombart]

After running:

nomad::build("reconhub/deployer", "D:/deployer")

my connection was interrupted ~10 min into the download. I tried completing the download using:

> nomad::pack("D:/deployer")
nomad: cran
[ download  ]  source
Skipping packages (type: source) for packages: ActuDistns
[ download  ]  binary: windows
Skipping packages (type: win.binary) for packages: ActuDistns, branchr, dibbler, distcrete, doMC, earlyR, epicontacts.ui, epitrix, gpclib, incidence.ui, outbreaker2, projections, recon.ui, recontools, RSVGTipsDevice, shinyHelpers
Error in download_files(url[i], destdir, labels = pkgs[i], progress = progress) : 
  Remove stale download file: D:/deployer/bin/windows/contrib/3.4/lpSolve_5.6.13.zip_dl

I remove the file manually (and successfully), but then it re-appears as soon as I run nomad::pack("D:/deployer").

I suspect this kind of issue will be frequent, so it would be good to be able to resume a stalled build.

[richfitz]

Testing this remains the challenge, so try

1. create a nomad archive
2. copy that to a series of directories
3. in each delete various components (some packages, git, r versions, etc)
4. compare against first directory and ensure that they match