Make `git` source's mutual exclusivity part of Pydantic schema
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### What needs to get done
The `git` source should encode the mutually exclusive properties of `source-tag`, `source-branch`, and `source-commit` into the Pydantic schema properly.
### Why it needs to get done
Spawned from the following TODO comment:
https://github.com/canonical/craft-parts/blob/a20b49d1b27ecd00ab74eb786d0f2403184adc97/craft_parts/sources/git_source.py#L55
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