Only use relative paths in lockfile (#6490)
For users who were using absolute paths in the `pyproject.toml`
previously, this is a behavior change: We now convert all absolute paths
in `path` entries to relative paths. Since i assume that no-one relies
on absolute path in their lockfiles - they are intended to be portable -
I'm tagging this as a bugfix.
Closes https://github.com/astral-sh/uv/pull/6438
Fixes https://github.com/astral-sh/uv/issues/6371
