Summarize package changes in `uv sync` json format output (#16981)
## Summary
Implement #16653 by making `uv sync --output-format=json` output
information about package changes.
## Test Plan
Additional tests to test the cases where there is no known package
version _may_ be beneficial but as the information used is the same as
the information used by the dry run logging now, I don't think that's
strictly necessary as those cases are tested.
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Co-authored-by: Liam <liam@scalzulli.com>
