Download hackage

# First get a list of all packages
cabal list --simple | tee packages.list

# Then compute the locations of their tar files
python3 -c "from fileinput import *; [print(\"https://hackage.haskell.org/package/\"+p+\"/\"+p+\"-\"+ver+\".tar.gz\") for (p,ver) in sorted(dict(map(str.split, input())).items())]" < packages.list | tee files.list

# If you are on windows make sure python didn't use CRLF:
# dos2unix files.list

# Download all the archives - 4 at a time. Some might fail, that is expected.
xargs -P4 wget -nv < files.list

# Do whatever you want afterwards. This was based on https://github.com/nh2/hackage-download, which sadly did not work for me.