### 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)

with TemporaryDirectory(prefix="venv") as tmp_dir:
            tmp_path = Path(tmp_dir)
            requirements_file_name = f"{tmp_dir}/requirements.txt"
            if not isinstance(self.requirements, str):
                requirements_file_contents = "\n".join(str(dependency) for dependency in self.requirements)
            else:
                requirements_file_contents = self.requirements
            if not self.system_site_packages and self.use_dill:
                requirements_file_contents += "\ndill"
            with open(requirements_file_name, "w") as file:
                file.write(requirements_file_contents)
            prepare_virtualenv(
                venv_directory=tmp_dir,
                python_bin=f"python{self.python_version}" if self.python_version else None,
                system_site_packages=self.system_site_packages,
                requirements_file_path=requirements_file_name,
                pip_install_options=self.pip_install_options,
            )
            python_path = tmp_path / "bin" / "python"
            result = self._execute_python_callable_in_subprocess(python_path, tmp_path)
            return result
    def _iter_serializable_context_keys(self):
        yield from self.BASE_SERIALIZABLE_CONTEXT_KEYS
        if self.system_site_packages or "apache-airflow" in self.requirements:
            yield from self.AIRFLOW_SERIALIZABLE_CONTEXT_KEYS
            yield from self.PENDULUM_SERIALIZABLE_CONTEXT_KEYS
        elif "pendulum" in self.requirements:
            yield from self.PENDULUM_SERIALIZABLE_CONTEXT_KEYS
class ExternalPythonOperator(_BasePythonVirtualenvOperator):