### 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)

self,
        *,
        python: str,
        python_callable: Callable,
        use_dill: bool = False,
        op_args: Collection[Any] | None = None,
        op_kwargs: Mapping[str, Any] | None = None,
        string_args: Iterable[str] | None = None,
        templates_dict: dict | None = None,
        templates_exts: list[str] | None = None,
        expect_airflow: bool = True,
        expect_pendulum: bool = False,
        skip_on_exit_code: int | Container[int] | None = None,
        **kwargs,
    ):
        if not python:
            raise ValueError("Python Path must be defined in ExternalPythonOperator")
        self.python = python
        self.expect_pendulum = expect_pendulum
        super().__init__(
            python_callable=python_callable,
            use_dill=use_dill,
            op_args=op_args,
            op_kwargs=op_kwargs,