Can't initialise vendored submodule

[Arnaud Spiwack]

Dear all,

I'm currently unable to build GHC's master branch as some of the submodules
(presumably used by Hadrian) are making git pretty unhappy (I've used
various variants of the command below, in a cleaned repository):

```console
$  git submodule update --init
error: submodule git dir
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian/vendored/Cabal'
is inside git dir
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian'
fatal: refusing to create/use
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian/vendored/Cabal'
in another submodule's git dir
Failed to clone 'hadrian/vendored/Cabal'. Retry scheduled
error: submodule git dir
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian/vendored/Cabal'
is inside git dir
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian'
fatal: refusing to create/use
'/home/aspiwack/projects/tweag/ghc/master/.git/modules/hadrian/vendored/Cabal'
in another submodule's git dir
Failed to clone 'hadrian/vendored/Cabal' a second time, aborting
```

Please advise on how to get unblocked.

And, if I'm allowed to conclude with a plea: please be careful not to abuse
Git's submodule system, it's wonky enough when used simply.

-- 
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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