**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
"imap": [],
    "jdbc": jdbc,
    "jenkins": jenkins,
    "jira": jira,
    "microsoft.azure": azure,
    "microsoft.mssql": mssql,
    "microsoft.winrm": winrm,
    "mongo": mongo,
    "mysql": mysql,
    "odbc": odbc,
    "openfaas": [],
    "opsgenie": [],
    "oracle": oracle,
    "pagerduty": pagerduty,
    "papermill": papermill,
    "plexus": plexus,
    "postgres": postgres,
    "presto": presto,
    "qubole": qubole,
    "redis": redis,
    "salesforce": salesforce,
    "samba": samba,
    "segment": segment,
    "sendgrid": sendgrid,
    "sftp": ssh,
    "singularity": singularity,
    "slack": slack,
    "snowflake": snowflake,
    "sqlite": [],
    "ssh": ssh,