yarn2nix: run nix-prefetch-git with --fetch-submodules

`pkgs.fetchgit` uses `fetchSubmodules = true;` by default, however
`nix-prefetch-git` doesn't. This means that hashes for a Git repository
with fetched submodules will be wrong in `yarn.nix`.

Considering that this got unnoticed before, it seems as if this case is
an exception to a certain degree.

An exemplary problem is the last `hedgedoc` update[1] where
`js-sequence-diagrams` - a Git repo with submodules - from upstream's
package.json caused a hash mismatch. This got unnoticed because
`nix-build --check` doesn't seem to reveal these issues for fixed-output
derivations.

[1] https://github.com/NixOS/nixpkgs/pull/139238

pkgs/development/tools/yarn2nix-moretea/yarn2nix/lib/generateNix.js

@@ -33,7 +33,7 @@ const { execFileSync } = require('child_process')
 
 function prefetchgit(url, rev) {
   return JSON.parse(
-    execFileSync("nix-prefetch-git", ["--rev", rev, url], {
+    execFileSync("nix-prefetch-git", ["--rev", rev, url, "--fetch-submodules"], {
       stdio: [ "ignore", "pipe", "ignore" ],
       timeout: 60000,
     })