initial support for using fetchGit

[cstrahan]

Generating the gomod2nix.toml appears to work, but I haven't tried building with this just yet.

The idea is that this should allow fetching private repositories (see https://github.com/tweag/gomod2nix/issues/3).

fetchGit does not work with short revs, so this:

• Tries to use the GitHub API where possible
• Falls back to cloning the repo in a temporary directory and then uses git rev-parse