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
+ hdfs
    + hive
    + mongo
    + mssql
    + mysql
    + pinot
    + postgres
    + presto
    + vertica
)
devel = [
    'beautifulsoup4~=4.7.1',
    'black',
    'blinker',
    'bowler',
    'click~=7.1',
    'contextdecorator;python_version<"3.4"',
    'coverage',
    'docutils',
    'flake8>=3.6.0',
    'flake8-colors',
    'flaky',
    'freezegun',
    'github3.py',
    'gitpython',
    'ipdb',
    'jira',
    'mongomock',
    'moto==1.3.14',