**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
"""
    Checks if package should be excluded.
    :param package: package name (beginning of it)
    :param exclusion_list: list of excluded packages
    :return: true if package should be excluded
    """
    return any(package.startswith(excluded_package) for excluded_package in exclusion_list)
devel_all = [
    package
    for package in devel_all
    if not is_package_excluded(package=package, exclusion_list=PACKAGES_EXCLUDED_FOR_ALL)
]
devel_ci = [
    package
    for package in devel_all
    if not is_package_excluded(
        package=package, exclusion_list=PACKAGES_EXCLUDED_FOR_CI + PACKAGES_EXCLUDED_FOR_ALL
    )
]
EXTRAS_REQUIREMENTS.update(
    {
        'all': devel_all,
        'devel_ci': devel_ci,
    }
)
INSTALL_REQUIREMENTS = [
    'alembic>=1.2, <2.0',
    'argcomplete~=1.10',
    'attrs>=20.0, <21.0',