**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**:



"""
    Test clearing a dag that has multiple ExternalTaskMarker.
    """
    agg_dag = dag_bag_multiple.get_dag("agg_dag")
    tis = run_tasks(dag_bag_multiple, execution_date=DEFAULT_DATE)
    session = settings.Session()
    try:
        qry = session.query(TaskInstance).filter(
            TaskInstance.state == State.NONE, TaskInstance.dag_id.in_(dag_bag_multiple.dag_ids)
        )
        assert agg_dag.clear(dag_bag=dag_bag_multiple) == len(tis) == qry.count() == 10
    finally:
        session.close()
@pytest.fixture
def dag_bag_head_tail():
    """
    Create a DagBag containing one DAG, with task "head" depending on task "tail" of the
    previous execution_date.
    20200501     20200502                 20200510
    +------+     +------+                 +------+
    | head |    -->head |    -->         -->head |
    |  |   |   / |  |   |   /           / |  |   |
    |  v   |  /  |  v   |  /           /  |  v   |
    | body | /   | body | /     ...   /   | body |
    |  |   |/    |  |   |/           /    |  |   |
    |  v   /     |  v   /           /     |  v   |
    | tail/|     | tail/|          /      | tail |