**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
'requests>=2.20.0, <3',
]
datadog = [
    'datadog>=0.14.0',
]
doc = [
    'sphinx>=2.1.2',
    'sphinx-argparse>=0.1.13',
    'sphinx-autoapi==1.0.0',
    'sphinx-copybutton',
    'sphinx-jinja~=1.1',
    'sphinx-rtd-theme>=0.1.6',
    'sphinxcontrib-httpdomain>=1.7.0',
    "sphinxcontrib-redoc>=1.6.0",
    "sphinxcontrib-spelling==5.2.1",
]
docker = [
    'docker~=3.0',
]
druid = [
    'pydruid>=0.4.1',
]
elasticsearch = [
    'elasticsearch>7, <7.6.0',
    'elasticsearch-dbapi==0.1.0',
    'elasticsearch-dsl>=5.0.0',
]
exasol = [
    'pyexasol>=0.5.1,<1.0.0',
]