**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
facebook = [
    'facebook-business>=6.0.2',
]
flask_oauth = [
    'Flask-OAuthlib>=0.9.1,<0.9.6',  
    'oauthlib!=2.0.3,!=2.0.4,!=2.0.5,<3.0.0,>=1.1.2',
    'requests-oauthlib==1.1.0',
]
google = [
    'PyOpenSSL',
    'google-ads>=4.0.0',
    'google-api-python-client>=1.6.0,<2.0.0',
    'google-auth>=1.0.0,<2.0.0',
    'google-auth-httplib2>=0.0.1',
    'google-cloud-automl>=0.4.0,<2.0.0',
    'google-cloud-bigquery-datatransfer>=0.4.0,<2.0.0',
    'google-cloud-bigtable>=1.0.0,<2.0.0',
    'google-cloud-container>=0.1.1,<2.0.0',
    'google-cloud-datacatalog>=0.5.0, <0.8',  
    'google-cloud-dataproc>=1.0.1,<2.0.0',
    'google-cloud-dlp>=0.11.0,<2.0.0',
    'google-cloud-kms>=1.2.1,<2.0.0',
    'google-cloud-language>=1.1.1,<2.0.0',
    'google-cloud-logging>=1.14.0,<2.0.0',
    'google-cloud-memcache>=0.2.0',
    'google-cloud-monitoring>=0.34.0,<2.0.0',
    'google-cloud-os-login>=1.0.0,<2.0.0',
    'google-cloud-pubsub>=1.0.0,<2.0.0',
    'google-cloud-redis>=0.3.0,<2.0.0',
    'google-cloud-secret-manager>=0.2.0,<2.0.0',