Log message argument missing for stager's .whl extra_package warning

In file sdks/python/apache_beam/runners/portability/stager.py, method apache_beam.runners.portability.stager.Stager._stage_extra_packages, package argument is missing for log message:
```

if os.path.basename(package).endswith('.whl'):
logging.warning(
'The .whl
package "%s" is provided in --extra_package. '
'This functionality is not officially supported.
Since wheel '
'packages are binary distributions, this package must be '
'binary-compatible
with the worker environment (e.g. Python 2.7 '
'running on an x64 Linux host).')

```

Imported from Jira [BEAM-7958](https://issues.apache.org/jira/browse/BEAM-7958). Original Jira may contain additional context.
Reported by: matjazs.

Open sdks/python/apache_beam/runners/portability/stager.py and inspect Stager._stage_extra_packages, especially the .whl warning. Confirm the warning receives the package value and that the completed message identifies the wheel package while retaining the existing warning text.

Written by the indexing model from the issue text.