**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
'devel': devel_minreq,
    'devel_hadoop': devel_hadoop,
    'doc': doc,
    'docker': docker,
    'druid': druid,  
    'elasticsearch': elasticsearch,
    'exasol': exasol,
    'facebook': facebook,
    'gcp': google,  
    'gcp_api': google,  
    'github_enterprise': flask_oauth,
    'google': google,
    'google_auth': flask_oauth,
    'grpc': grpc,
    'hashicorp': hashicorp,
    'hdfs': hdfs,  
    'hive': hive,  
    'jdbc': jdbc,
    'jira': jira,
    'kerberos': kerberos,
    'kubernetes': kubernetes,  
    'ldap': ldap,
    "microsoft.azure": azure,
    "microsoft.mssql": mssql,
    "microsoft.winrm": winrm,
    'mongo': mongo,
    'mssql': mssql,  
    'mysql': mysql,
    'odbc': odbc,
    'oracle': oracle,