**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)


"""Required by TaskMixin"""
        return list(self.get_leaves())
    def get_roots(self) -> Generator[BaseOperator, None, None]:
        """
        Returns a generator of tasks that are root tasks, i.e. those with no upstream
        dependencies within the TaskGroup.
        """
        for task in self:
            if not any(self.has_task(parent) for parent in task.get_direct_relatives(upstream=True)):
                yield task
    def get_leaves(self) -> Generator[BaseOperator, None, None]:
        """
        Returns a generator of tasks that are leaf tasks, i.e. those with no downstream
        dependencies within the TaskGroup
        """
        for task in self:
            if not any(self.has_task(child) for child in task.get_direct_relatives(upstream=False)):
                yield task
    def child_id(self, label):
        """
        Prefix label with group_id if prefix_group_id is True. Otherwise return the label
        as-is.
        """
        if self.prefix_group_id and self.group_id:
            return f"{self.group_id}.{label}"
        return label
    @property
    def upstream_join_id(self) -> str: