**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
"vertica": vertica,
    "yandex": yandexcloud,
    "zendesk": zendesk,
}
EXTRAS_REQUIREMENTS: Dict[str, List[str]] = {
    'all_dbs': all_dbs,
    'amazon': amazon,
    'apache.atlas': atlas,
    'apache.beam': apache_beam,
    "apache.cassandra": cassandra,
    "apache.druid": druid,
    "apache.hdfs": hdfs,
    "apache.hive": hive,
    "apache.kylin": kylin,
    "apache.pinot": pinot,
    "apache.presto": presto,
    "apache.spark": spark,
    "apache.webhdfs": webhdfs,
    'async': async_packages,
    'atlas': atlas,  
    'aws': amazon,  
    'azure': azure,  
    'cassandra': cassandra,  
    'celery': celery,
    'cgroups': cgroups,
    'cloudant': cloudant,
    'cncf.kubernetes': kubernetes,
    'dask': dask,
    'databricks': databricks,
    'datadog': datadog,