**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
"""Perform the Airflow package setup."""
    install_providers_from_sources = os.getenv('INSTALL_PROVIDERS_FROM_SOURCES')
    exclude_patterns = (
        []
        if install_providers_from_sources and install_providers_from_sources == 'true'
        else ['airflow.providers', 'airflow.providers.*']
    )
    write_version()
    if not install_providers_from_sources:
        for key, value in EXTRAS_PROVIDERS_PACKAGES.items():
            EXTRAS_REQUIREMENTS[key].extend(
                [get_provider_package_from_package_id(package_name) for package_name in value]
            )
    packages_to_install = (
        find_namespace_packages(include=['airflow*'], exclude=exclude_patterns)
        if install_providers_from_sources
        else find_packages(include=['airflow*'], exclude=exclude_patterns)
    )
    setup(
        name='apache-airflow',
        description='Programmatically author, schedule and monitor data pipelines',
        long_description=long_description,
        long_description_content_type='text/markdown',
        license='Apache License 2.0',
        version=version,