**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
"apache.webhdfs": ["apache.hdfs"],
    'async': [],
    'atlas': [],  
    'aws': ["amazon"],  
    'azure': ["microsoft.azure"],  
    'cassandra': ["apache.cassandra"],  
    'celery': ["celery"],
    'cgroups': [],
    'cloudant': ["cloudant"],
    'cncf.kubernetes': ["cncf.kubernetes"],
    'dask': ["dask"],
    'databricks': ["databricks"],
    'datadog': ["datadog"],
    'devel': ["cncf.kubernetes", "mysql"],
    'devel_hadoop': ["apache.hdfs", "apache.hive", "presto"],
    'doc': [],
    'docker': ["docker"],
    'druid': ["apache.druid"],  
    'elasticsearch': ["elasticsearch"],
    'exasol': ["exasol"],
    'facebook': ["facebook"],
    'gcp': ["google"],  
    'gcp_api': ["google"],  
    'github_enterprise': [],
    'google': ["google"],
    'google_auth': [],
    'grpc': ["grpc"],
    'hashicorp': ["hashicorp"],
    'hdfs': ["apache.hdfs"],  
    'hive': ["apache.hive"],