**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
#
#
#
"""Setup.py for the Airflow project."""
import logging
import os
import subprocess
import sys
import unittest
from os.path import dirname
from textwrap import wrap
from typing import Dict, Iterable, List
from setuptools import Command, find_namespace_packages, find_packages, setup
logger = logging.getLogger(__name__)
version = '2.0.0b2'
PY3 = sys.version_info[0] == 3
my_dir = dirname(__file__)
try:
    with open(os.path.join(my_dir, 'README.md'), encoding='utf-8') as f:
        long_description = f.read()
except FileNotFoundError:
    long_description = ''
def airflow_test_suite():
    """Test suite for Airflow tests"""
    test_loader = unittest.TestLoader()
    test_suite = test_loader.discover(os.path.join(my_dir, 'tests'), pattern='test_*.py')
    return test_suite
class CleanCommand(Command):