`metadata_directory` already contains dist-info directory (#10005)
From PEP 517:
```python
def prepare_metadata_for_build_wheel(metadata_directory, config_settings=None):
...
```
> Must create a .dist-info directory containing wheel metadata inside
the specified metadata_directory (i.e., creates a directory like
{metadata_directory}/{package}-{version}.dist-info/).
```python
def build_wheel(wheel_directory, config_settings=None, metadata_directory=None):
...
```
> If the build frontend has previously called
prepare_metadata_for_build_wheel and depends on the wheel resulting from
this call to have metadata matching this earlier call, then it should
provide the path to the created .dist-info directory as the
metadata_directory argument.
Notice that the `metadata_directory` is different for the both hooks:
For `prepare_metadata_for_build_wheel` is doesn't contain the
`.dist-info` directory as final segment, for `build_wheel` it does.
Previously, the code assumed that both directories didn't contain the
`.dist-info` for both cases.
Checked with:
```
maturin build
uv init test-uv-build-backend --build-backend uv
cd test-uv-build-backend
uv build --sdist --preview
cd ..
UV_PREVIEW=1 pip install test-uv-build-backend/dist/test_uv_build_backend-0.1.0.tar.gz --no-index --find-links target/wheels/ -v --no-cache-dir
```
Fixes #9969
