### 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)

op_kwargs=op_kwargs,
            templates_dict=templates_dict,
            templates_exts=templates_exts,
            **kwargs,
        )
        self.string_args = string_args or []
        self.use_dill = use_dill
        self.pickling_library = dill if self.use_dill else pickle
        self.expect_airflow = expect_airflow
        self.skip_on_exit_code = (
            skip_on_exit_code
            if isinstance(skip_on_exit_code, Container)
            else [skip_on_exit_code]
            if skip_on_exit_code
            else []
        )
    @abstractmethod
    def _iter_serializable_context_keys(self):
        pass
    def execute(self, context: Context) -> Any:
        serializable_keys = set(self._iter_serializable_context_keys())
        serializable_context = context_copy_partial(context, serializable_keys)
        return super().execute(context=serializable_context)
    def get_python_source(self):
        """Return the source of self.python_callable."""
        return dedent(inspect.getsource(self.python_callable))
    def _write_args(self, file: Path):
        if self.op_args or self.op_kwargs:
            file.write_bytes(self.pickling_library.dumps({"args": self.op_args, "kwargs": self.op_kwargs}))
    def _write_string_args(self, file: Path):