### Apache Airflow version

2.3.0

### What happened

When using a `PythonVirtualenvOperator` it is checked if `virtualenv` is installed by 
`if not shutil.which("virtualenv"):`
https://github.com/apache/airflow/blob/a1679be85aa49c0d6a7ba2c31acb519a5bcdf594/airflow/operators/python.py#L398
Actually, this expression checks if `virtualenv` is on PATH. If Airflow is installed in a virtual environment itself and `virtualenv` is not installed in the environment the check might pass but `virtualenv` cannot be used as it is not present in the environment.

### What you think should happen instead

It should be checked if `virtualenv` is actually available in the environment.

```python
if importlib.util.find_spec("virtualenv") is None:
    raise AirflowException('PythonVirtualenvOperator requires virtualenv, please install it.')
```
https://stackoverflow.com/a/14050282

### How to reproduce

_No response_

### Operating System

Ubuntu 20.04

### Versions of Apache Airflow Providers

_No response_

### Deployment

Virtualenv installation

### Deployment details

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

"""Default arguments for specific Operator."""
        return kwargs
    def create_dag_run(self) -> DagRun:
        return self.dag.create_dagrun(
            state=DagRunState.RUNNING,
            start_date=self.dag_maker.start_date,
            session=self.dag_maker.session,
            execution_date=self.default_date,
            run_type=DagRunType.MANUAL,
        )
    def create_ti(self, fn, **kwargs) -> TI:
        """Create TaskInstance for class defined Operator."""
        return self.ti_maker(
            self.opcls,
            python_callable=fn,
            **self.default_kwargs(**kwargs),
            dag_id=self.dag_id,
            task_id=self.task_id,
            execution_date=self.default_date,
        )
    def run_as_operator(self, fn, **kwargs):
        """Run task by direct call ``run`` method."""
        with self.dag:
            task = self.opcls(task_id=self.task_id, python_callable=fn, **self.default_kwargs(**kwargs))
        task.run(start_date=self.default_date, end_date=self.default_date)
        return task
    def run_as_task(self, fn, return_ti=False, **kwargs):