WIP: support git submodules for flakes

I want git submodules badly, so here is my equally bad attempt on adding
support for them in flakes.

TODO:
- This now unconditionally adds submodules for all flake inputs. Do we
  want that?
- This currently makes it equivalent to appending `?submodules=1` to the
  url. Do we want that?
- Figure out how this can be done for `self`.
- Add a test for the behaviour

Fixes: NixOS#4357

src/libexpr/flake/flakeref.cc

@@ -144,6 +144,7 @@ std::pair<FlakeRef, std::string> parseFlakeRefWithFragment(
 
                     if (pathExists(flakeRoot + "/.git/shallow"))
                         parsedURL.query.insert_or_assign("shallow", "1");
+                    parsedURL.query.insert_or_assign("submodule", "1"); // TODO: Base this on a condition
 
                     return std::make_pair(
                         FlakeRef(Input::fromURL(parsedURL), get(parsedURL.query, "dir").value_or("")),

src/libfetchers/git.cc

@@ -45,6 +45,8 @@ struct GitInputScheme : InputScheme
                 attrs.emplace(name, value);
             else if (name == "shallow")
                 attrs.emplace(name, Explicit<bool> { value == "1" });
+            else if (name == "submodules")
+                attrs.emplace(name, Explicit<bool> { value == "1" });
             else
                 url2.query.emplace(name, value);
         }