**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


"""List of nodes directly upstream"""
        if not self.dag:
            raise AirflowException(f"Operator {self} has not been assigned to a DAG yet")
        return [self.dag.get_task(tid) for tid in self.upstream_task_ids]
    def get_direct_relative_ids(self, upstream: bool = False) -> set[str]:
        """
        Get set of the direct relative ids to the current task, upstream or
        downstream.
        """
        if upstream:
            return self.upstream_task_ids
        else:
            return self.downstream_task_ids
    def get_direct_relatives(self, upstream: bool = False) -> Iterable[DAGNode]:
        """
        Get list of the direct relatives to the current task, upstream or
        downstream.
        """
        if upstream:
            return self.upstream_list
        else:
            return self.downstream_list
    def serialize_for_task_group(self) -> tuple[DagAttributeTypes, Any]:
        """This is used by TaskGroupSerialization to serialize a task group's content."""
        raise NotImplementedError()