**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
'cached_property~=1.5',
    
    'cattrs>=1.0, <1.1.0;python_version<="3.6"',
    'cattrs>=1.0, <2.0;python_version>"3.6"',
    'colorlog==4.0.2',
    'connexion[swagger-ui,flask]>=2.6.0,<3',
    'croniter>=0.3.17, <0.4',
    'cryptography>=0.9.3',
    'dill>=0.2.2, <0.4',
    'flask>=1.1.0, <2.0',
    'flask-appbuilder~=3.1.1',
    'flask-caching>=1.5.0, <2.0.0',
    'flask-login>=0.3, <0.5',
    'flask-swagger==0.2.13',
    'flask-wtf>=0.14.3, <0.15',
    'funcsigs>=1.0.0, <2.0.0',
    'graphviz>=0.12',
    'gunicorn>=19.5.0, <20.0',
    'iso8601>=0.1.12',
    'jinja2>=2.10.1, <2.12.0',
    'json-merge-patch==0.2',
    'jsonschema~=3.0',
    'lazy_object_proxy~=1.3',
    'lockfile>=0.12.2',
    'markdown>=2.5.2, <4.0',
    'markupsafe>=1.1.1, <2.0',
    'marshmallow-oneofschema>=2.0.1',
    'pandas>=0.17.1, <2.0',
    'pendulum~=2.0',
    'pep562~=1.0;python_version<"3.7"',