**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
]
azure = [
    'azure-batch>=8.0.0',
    'azure-cosmos>=3.0.1,<4',
    'azure-datalake-store>=0.0.45',
    'azure-identity>=1.3.1',
    'azure-keyvault>=4.1.0',
    'azure-kusto-data>=0.0.43,<0.1',
    'azure-mgmt-containerinstance>=1.5.0,<2.0',
    'azure-mgmt-datalake-store>=0.5.0',
    'azure-mgmt-resource>=2.2.0',
    'azure-storage>=0.34.0, <0.37.0',
    'azure-storage-blob<12.0',
]
cassandra = [
    'cassandra-driver>=3.13.0,<3.21.0',
]
celery = [
    'celery~=4.4.2',
    'flower>=0.7.3, <1.0',
    'vine~=1.3',  
]
cgroups = [
    'cgroupspy>=0.1.4',
]
cloudant = [
    'cloudant>=2.0',
]
dask = ['cloudpickle>=1.4.1, <1.5.0', 'distributed>=2.11.1, <2.20']
databricks = [