selfhost/typechecker: Prelude structs aren't included in root module

[cg-jl]

After debugging a failed index to an array that happens when trying to typecheck the first call to flag in selfhost/main.jakt (line 26), I injected a couple of print statements and found that, to my surprise, the structures array in the module with module ID 0 (i.e the root module) is empty. Since main.jakt doesn't define any structs or classes it would be correct if it was an imported module, but since it's the root module then it should've had the prelude types like Optional, Array...

If you're interested, the access comes from this code: https://github.com/SerenityOS/jakt/blob/d6e538a29f7860f973ef62daf98ac2fe9da8e777/selfhost/typechecker.jakt#L2540-L2544 that gets the structure so it can get the name in case it has to produce an error.

[cg-jl]

A reduced sample:

function a(args: [String]) {}

function main() {
    let args = ["1", "2", "3"]
    a(args)
}

[lanmonster]

fixed as of 1ea81c6