fix: skip cloning badly defined submodules

eg with this dependency
```
pyscf = { git = "https://github.com/pyscf/pyscf", tag = "v2.0.1"}
```
the cloning of submodules fails - because that project has messed up and not committed the file saying what revision they want of the `doc` submodule.

Legacy git client just silently carries on, so I've done the same here.