**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
'pyodbc',
]
oracle = [
    'cx_Oracle>=5.1.2',
]
pagerduty = [
    'pdpyras>=4.1.2,<5',
]
papermill = [
    'papermill[all]>=1.2.1',
    'nteract-scrapbook[all]>=0.3.1',
]
password = [
    'bcrypt>=2.0.0',
    'flask-bcrypt>=0.7.1',
]
pinot = [
    'pinotdb==0.1.1',
]
plexus = [
    'arrow>=0.16.0',
]
postgres = [
    'psycopg2-binary>=2.7.4',
]
presto = ['presto-python-client>=0.7.0,<0.8']
qubole = [
    'qds-sdk>=1.10.4',
]
rabbitmq = [