**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
"""
    Compile and build the frontend assets using yarn and webpack.
    Registered as cmdclass in setup() so it can be called with ``python setup.py compile_assets``.
    """
    description = "Compile and build the frontend assets"
    user_options = []  
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):
        """Set final values for options."""
    def run(self):  
        """Run a command to compile and build assets."""
        subprocess.check_call('./airflow/www/compile_assets.sh')
class ListExtras(Command):
    """
    List all available extras
    Registered as cmdclass in setup() so it can be called with ``python setup.py list_extras``.
    """
    description = "List available extras"
    user_options = []  
    def initialize_options(self):
        """Set default values for options."""
    def finalize_options(self):