Allow symlinks with `--find-links` (#5323)
## Summary
In my setup, I have a directory of wheels symlinked from different
directories. I can point `--find-links` at it with `pip` and it works
but not `uv`.
Currently, `uv` checks if a candidate file `is_file` which is for
regular files. By also checking `is_symlink` I was able to install a
symlinked wheel. I'm not *exactly* sure where, but some other place is
eventually resolving the absolute path of the wheel. (`uv`? The OS?)
## Test Plan
Manually tested - I didn't see any tests for `FlatIndexClient` in the
`uv-client` crate.
```
mkdir /tmp/a /tmp/b # Create a directory of wheels (/tmp/a) and a directory of symlinked wheels (/tmp/b)
cp test-0.0.1-py3-none-any.whl /tmp/a # Add a wheel to the directory of wheels
ln -s /tmp/a/test-0.0.1-py3-none-any.whl /tmp/b/ # Create a symlink to that wheel
uv pip install test --find-links /tmp/b # Install pointing at the symlinked wheels directory
```
