### 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)

from airflow import __version__ as airflow_version
        try:
            result = subprocess.check_output(
                [self.python, "-c", "from airflow import __version__; print(__version__)"],
                text=True,
                
                env={**os.environ, "_AIRFLOW__AS_LIBRARY": "true"},
            )
            target_airflow_version = result.strip()
            if target_airflow_version != airflow_version:
                raise AirflowConfigException(
                    f"The version of Airflow installed for the {self.python}("
                    f"{target_airflow_version}) is different than the runtime Airflow version: "
                    f"{airflow_version}. Make sure your environment has the same Airflow version "
                    f"installed as the Airflow runtime."
                )
            return target_airflow_version
        except Exception as e:
            if self.expect_airflow:
                self.log.warning("When checking for Airflow installed in venv got %s", e)
                self.log.warning(
                    f"This means that Airflow is not properly installed by  "
                    f"{self.python}. Airflow context keys will not be available. "
                    f"Please Install Airflow {airflow_version} in your environment to access them."
                )
            return None
class ExternalBranchPythonOperator(ExternalPythonOperator, SkipMixin):