nix-prefetch-scripts: make nix-prefetch-git report fetchSubmodules in its JSON output

Previously, nix-prefetch-git would report the same JSON whether submodules were being fetched or not; with this change, the --fetch-submodules option will cause the JSON output to include "fetchSubmodules": true, so that fetchgit (builtins.fromJSON (builtins.readFile ./path/to/output.json)) will work.