Create fork networks for forks for which the source was deleted.
That way we can join forks-of-forks into the same network even if their original source was deleted. Consider the following: `awesome-oss/badger` is forked to `coolstuff/badger`, which is forked to `user-a/badger` which in turn is forked to `user-b/badger`. When `awesome-oss/badger` is deleted, we will now create a fork network with `coolstuff/badger` as the root. The `user-a/badger` and `user-b/badger` projects will be added to the network.
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