**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
'all': list(PROVIDERS_REQUIREMENTS.keys()),
    
    
    'devel_ci': list(PROVIDERS_REQUIREMENTS.keys()),
    'all_dbs': [
        "apache.cassandra",
        "apache.druid",
        "apache.hdfs",
        "apache.hive",
        "apache.pinot",
        "cloudant",
        "exasol",
        "mongo",
        "microsoft.mssql",
        "mysql",
        "postgres",
        "presto",
        "vertica",
    ],
    'amazon': ["amazon"],
    'apache.atlas': [],
    'apache.beam': [],
    "apache.cassandra": ["apache.cassandra"],
    "apache.druid": ["apache.druid"],
    "apache.hdfs": ["apache.hdfs"],
    "apache.hive": ["apache.hive"],
    "apache.kylin": ["apache.kylin"],
    "apache.pinot": ["apache.pinot"],
    "apache.presto": ["apache.presto"],
    "apache.spark": ["apache.spark"],