**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)


#
#
#
"""
A TaskGroup is a collection of closely related tasks on the same DAG that should be grouped
together when the DAG is displayed graphically.
"""
from __future__ import annotations
import copy
import functools
import operator
import re
import weakref
from typing import TYPE_CHECKING, Any, Generator, Iterator, Sequence
from airflow.compat.functools import cache
from airflow.exceptions import (
    AirflowDagCycleException,
    AirflowException,
    DuplicateTaskIdFound,
    TaskAlreadyInTaskGroup,
)
from airflow.models.taskmixin import DAGNode, DependencyMixin
from airflow.serialization.enums import DagAttributeTypes
from airflow.utils.helpers import validate_group_key
if TYPE_CHECKING:
    from sqlalchemy.orm import Session
    from airflow.models.abstractoperator import AbstractOperator
    from airflow.models.baseoperator import BaseOperator
    from airflow.models.dag import DAG
    from airflow.models.expandinput import ExpandInput