Fix duplications in results cache

[asafgabai]

• All tests passed. If this feature is not already covered by the tests, I added new tests.

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Fix duplications of VulnerabilityNodes in the cache, that caused errors while trying to read it.
Usually, a VulnerabilityNode is written to the cache file once: as a child of a DescriptorFileTreeNode.
If there's an ApplicableIssueNode pointing to it, then this ApplicableIssueNode contains a field with the VulnerabilityNode's UUID.
But sometimes, if the ApplicableIssueNode pointing to this VulnerabilityNode has been written before the VulnerabilityNode, then the VulnerabilityNode is written inside the ApplicableIssueNode (instead of just its UUID), and later when the DescriptorFileTreeNode (that contains this VulnerabilityNode) is written, then the VulnerabilityNode is written again.
Both times the VulnerabilityNode is written with the same UUID (because it's the same object), so when the cache is read, a parsing error is thrown, saying there are multiple objects with the same UUID.
To fix that, I added an annotation that enforces writing the VulnerabilityNode, that's inside the ApplicableIssueNode, as a UUID only.