Fix transitive Git path dependencies in lockfiles (#19269)
## Summary
We now preserve source identity for transitive path dependencies
discovered while lowering metadata from Git-sourced packages. If a
dependency resolves to a directory inside the same Git checkout, we lock
it as the corresponding Git source with a `subdirectory` instead of
serializing the absolute checkout path.
Closes https://github.com/astral-sh/uv/issues/19152.
