**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
devel_minreq = cgroups + devel + doc + kubernetes + mysql + password
devel_hadoop = devel_minreq + hdfs + hive + kerberos + presto + webhdfs
PROVIDERS_REQUIREMENTS: Dict[str, Iterable[str]] = {
    "amazon": amazon,
    "apache.cassandra": cassandra,
    "apache.druid": druid,
    "apache.hdfs": hdfs,
    "apache.hive": hive,
    "apache.kylin": kylin,
    "apache.livy": [],
    "apache.pig": [],
    "apache.pinot": pinot,
    "apache.spark": spark,
    "apache.sqoop": [],
    "celery": celery,
    "cloudant": cloudant,
    "cncf.kubernetes": kubernetes,
    "databricks": databricks,
    "datadog": datadog,
    "dingding": [],
    "discord": [],
    "docker": docker,
    "elasticsearch": [],
    "exasol": exasol,
    "facebook": facebook,
    "ftp": [],
    "google": google,
    "grpc": grpc,
    "hashicorp": hashicorp,
    "http": [],