**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
'python-jenkins>=1.0.0',
]
jira = [
    'JIRA>1.0.7',
]
kerberos = [
    'pykerberos>=1.1.13',
    'requests_kerberos>=0.10.0',
    'thrift_sasl>=0.2.0',
]
kubernetes = [
    'cryptography>=2.0.0',
    'kubernetes>=3.0.0, <12.0.0',
]
kylin = ['kylinpy>=2.6']
ldap = [
    'ldap3>=2.5.1',
]
mongo = [
    'dnspython>=1.13.0,<2.0.0',
    'pymongo>=3.6.0',
]
mssql = [
    'pymssql~=2.1,>=2.1.5',
]
mysql = [
    'mysql-connector-python>=8.0.11, <=8.0.18',
    'mysqlclient>=1.3.6,<1.4',
]
odbc = [