**Apache Airflow version**: 2.0.0b2

**What happened**:

Starting in 2.0.0b2, extras that aren't also providers like `statsd` prevents Airflow from being installed.

**How to reproduce it**:
```
$ docker run --rm -ti python:3.6 bash
# pip install apache-airflow[statsd]==2.0.0b2
Collecting apache-airflow[statsd]==2.0.0b2                 
  Downloading apache_airflow-2.0.0b2-py3-none-any.whl (4.5 MB)                    
...
Collecting statsd<4.0,>=3.3.0; extra == "statsd"          
  Downloading statsd-3.3.0-py2.py3-none-any.whl (11 kB)
ERROR: Could not find a version that satisfies the requirement apache-airflow-providers-statsd; extra == "statsd" (from apache-airflow[statsd]==2.0.0b2) (from versions: none)         
ERROR: No matching distribution found for apache-airflow-providers-statsd; extra == "statsd" (from apache-airflow[statsd]==2.0.0b2)
```

I believe this is from https://github.com/apache/airflow/pull/12233
setup.py
packages=packages_to_install,
        package_data={
            'airflow': ['py.typed'],
            '': [
                'airflow/alembic.ini',
                "airflow/git_version",
                "*.ipynb",
                "airflow/providers/cncf/kubernetes/example_dags/*.yaml",
            ],
            'airflow.api_connexion.openapi': ['*.yaml'],
            'airflow.serialization': ["*.json"],
        },
        include_package_data=True,
        zip_safe=False,
        entry_points={
            "console_scripts": [
                "airflow = airflow.__main__:main",
            ],
        },
        install_requires=INSTALL_REQUIREMENTS,
        setup_requires=[
            'bowler',
            'docutils',
            'gitpython',
            'setuptools',
            'wheel',
        ],
        extras_require=EXTRAS_REQUIREMENTS,
        classifiers=[
            'Development Status :: 5 - Production/Stable',