Get Nix Flake to include git submodule

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3

What’s the input url supposed to be for a submodule in the flake’s directory? Checking out from remote works (very inefficient), and this specific combination of:

url = "file:///[snip]/submodule?submodules=1";
type = "git";

works, but these failed:

inputs.submodule = {
    flake = false;
    url = "git+file:///[snip]/submodule?submodules=1";
}; # Fails w/ `error: opening file '[snip]/submodule/.git/config': Not a directory`

inputs.submodule = {
    flake = false;
    url = "git+path:submodule?submodules=1";
}; # Unsupported

inputs.submodule = {
    flake = false;
    url = "path:submodule?submodules=1";
    type = "git";
}; # Unsupported

I’d rather not have to specify the absolute path to the submodule.