**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): v1.16.3

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release): Debian GNU/Linux 10 (buster)
- **Kernel** (e.g. `uname -a`): Linux airflow 3.10.0-1127.10.1.el7.x86_64 #1 SMP Tue May 26 15:05:43 EDT 2020 x86_64 GNU/Linux
- **Install tools**:
- **Others**:

**What happened**:

ExternalTaskSensor with check_existence=True referencing an external DAG inside a .zip file raises the following exception:
```
ERROR - The external DAG dag_a /opt/airflow-data/dags/my_dags.zip/dag_a.py was deleted.
Traceback (most recent call last):
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1086, in _run_raw_task
    self._prepare_and_execute_task_with_callbacks(context, task)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1260, in _prepare_and_execute_task_with_callbacks
    result = self._execute_task(context, task_copy)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/models/taskinstance.py", line 1300, in _execute_task
    result = task_copy.execute(context=context)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/sensors/base.py", line 228, in execute
    while not self.poke(context):
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/sensors/external_task.py", line 159, in poke
    self._check_for_existence(session=session)
  File "/home/airflow/.local/lib/python3.7/site-packages/airflow/sensors/external_task.py", line 184, in _check_for_existence
    raise AirflowException(f'The external DAG {self.external_dag_id} was deleted.')
airflow.exceptions.AirflowException: The external DAG dag_a /opt/airflow-data/dags/my_dags.zip/dag_a.py was deleted.
```

**What you expected to happen**:

The existence check should PASS.

**How to reproduce it**:

1. Create a file *dag_a.py* with the following contents:
```
from airflow import DAG
from airflow.operators.dummy import DummyOperator
from airflow.utils.timezone import datetime

DEFAULT_DATE = datetime(2015, 1, 1)
with DAG("dag_a", start_date=DEFAULT_DATE, schedule_interval="@daily") as dag:
    task_a = DummyOperator(task_id="task_a", dag=dag)
```

2. Create a file *dag_b.py* with contents:
```
from airflow import DAG
from airflow.operators.dummy import DummyOperator
from airflow.sensors.external_task import ExternalTaskSensor
from airflow.utils.timezone import datetime

DEFAULT_DATE = datetime(2015, 1, 1)

with DAG("dag_b", start_date=DEFAULT_DATE, schedule_interval="@daily") as dag:
    sense_a = ExternalTaskSensor(
        task_id="sense_a",
        external_dag_id="dag_a",
        external_task_id="task_a",
        check_existence=True
    )
    task_b = DummyOperator(task_id="task_b", dag=dag)
    sense_a >> task_b
```

3. `zip my_dags.zip dag_a.py dag_b.py`
4. Load *my_dags.zip* into airflow and run *dag_b*
5. Task *sense_a* will fail with exception above.

**Anything else we need to know**:



the external task skips.
    :param external_dag_id: The dag_id that contains the task you want to
        wait for
    :param external_task_id: The task_id that contains the task you want to
        wait for.
    :param external_task_ids: The list of task_ids that you want to wait for.
        If ``None`` (default value) the sensor waits for the DAG. Either
        external_task_id or external_task_ids can be passed to
        ExternalTaskSensor, but not both.
    :param allowed_states: Iterable of allowed states, default is ``['success']``
    :param failed_states: Iterable of failed or dis-allowed states, default is ``None``
    :param execution_delta: time difference with the previous execution to
        look at, the default is the same logical date as the current task or DAG.
        For yesterday, use [positive!] datetime.timedelta(days=1). Either
        execution_delta or execution_date_fn can be passed to
        ExternalTaskSensor, but not both.
    :param execution_date_fn: function that receives the current execution's logical date as the first
        positional argument and optionally any number of keyword arguments available in the
        context dictionary, and returns the desired logical dates to query.
        Either execution_delta or execution_date_fn can be passed to ExternalTaskSensor,
        but not both.
    :param check_existence: Set to `True` to check if the external task exists (when
        external_task_id is not None) or check if the DAG to wait for exists (when
        external_task_id is None), and immediately cease waiting if the external task
        or DAG does not exist (default value: False).
    """
    template_fields = ["external_dag_id", "external_task_id", "external_task_ids"]
    ui_color = "#19647e"
    operator_extra_links = [ExternalDagLink()]
    def __init__(