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
'pykerberos>=1.1.13',
    'requests_kerberos>=0.10.0',
    'thrift_sasl>=0.2.0',
]
kubernetes = [
    'cryptography>=2.0.0',
    'kubernetes>=3.0.0, <12.0.0',
]
kylin = ['kylinpy>=2.6']
ldap = [
    'ldap3>=2.5.1',
]
mongo = [
    'dnspython>=1.13.0,<2.0.0',
    'pymongo>=3.6.0',
]
mssql = [
    'pymssql~=2.1,>=2.1.5',
]
mysql = [
    'mysql-connector-python>=8.0.11, <=8.0.18',
    'mysqlclient>=1.3.6,<1.4',
]
odbc = [
    'pyodbc',
]
oracle = [
    'cx_Oracle>=5.1.2',
]
pagerduty = [