clone dependencies from the previous checkout if the checkout artifact is already present

The following discussion from !5 (merged) should be addressed:

@maszyman started a discussion: (+1 comment)

I wonder if we should clone also doc['config']['projects'][...]['dependencies'], because if the artifact is already present, the list returned by dependencies() will be empty (as it is set by set_dependencies() in lb/nightly/functions/rpc.py::checkout).

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