Suggestion: switch to using Git directly

[athas]

smlpkg is closely based on the Futhark package manager, which recently switched to using git directly instead of downloading from GitHub/GitLab websites. This means arbitrary Git hosting sites (or Git over random SSH connections) can be supported. Most importantly, private repositories also work. Discussion here: diku-dk/futhark#1835

Since the implementation of smlpkg is still very similar to the original Haskell code, such a switch would not be difficult. In fact, it turns out that it is less code in total.