**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
'gitpython',
    'ipdb',
    'jira',
    'mongomock',
    'moto==1.3.14',  
    
    'parameterized',
    'paramiko',
    'pipdeptree',
    'pre-commit',
    'pylint==2.5.3',
    'pysftp',
    'pytest',
    'pytest-cov',
    'pytest-instafail',
    'pytest-rerunfailures',
    'pytest-timeouts',
    'pytest-xdist',
    'pywinrm',
    'qds-sdk>=1.9.6',
    'requests_mock',
    'setuptools',
    'testfixtures',
    'wheel',
    'yamllint',
]
if PY3:
    devel += ['mypy==0.770']
else:
    devel += ['unittest2']