When you install Aiflow with an extra, the required provider packages should be installed automatically.

For example:

`pip instal apache-airflow[google]` should install apache-airlfow-providers-google package.
setup.py
doc = [
    'sphinx>=2.1.2',
    'sphinx-argparse>=0.1.13',
    'sphinx-autoapi==1.0.0',
    'sphinx-copybutton',
    'sphinx-jinja~=1.1',
    'sphinx-rtd-theme>=0.1.6',
    'sphinxcontrib-httpdomain>=1.7.0',
    "sphinxcontrib-redoc>=1.6.0",
    "sphinxcontrib-spelling==5.2.1",
]
docker = [
    'docker~=3.0',
]
druid = [
    'pydruid>=0.4.1',
]
elasticsearch = [
    'elasticsearch>7, <7.6.0',
    'elasticsearch-dbapi==0.1.0',
    'elasticsearch-dsl>=5.0.0',
]
exasol = [
    'pyexasol>=0.5.1,<1.0.0',
]
facebook = [
    'facebook-business>=6.0.2',
]
flask_oauth = [
    'Flask-OAuthlib>=0.9.1,<0.9.6',