[AIRFLOW-5807] Move SFTP from contrib to providers.

[TobKed]

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[mik-laj]

@TobKed Travis is sad. Can you fix it?

[TobKed]

Rebased after fix for broken master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6f5c8cf). Click here to learn what that means.
The diff coverage is 93.26%.

@@            Coverage Diff            @@
##             master    #6464   +/-   ##
=========================================
  Coverage          ?   84.56%           
=========================================
  Files             ?      672           
  Lines             ?    37944           
  Branches          ?        0           
=========================================
  Hits              ?    32086           
  Misses            ?     5858           
  Partials          ?        0

Impacted Files	Coverage Δ
airflow/contrib/hooks/sftp_hook.py	100% <100%> (ø)
airflow/operators/gcs_to_sftp.py	100% <100%> (ø)
airflow/contrib/operators/sftp_operator.py	100% <100%> (ø)
...ow/providers/google/cloud/operators/sftp_to_gcs.py	100% <100%> (ø)
airflow/contrib/sensors/sftp_sensor.py	100% <100%> (ø)
airflow/providers/sftp/sensors/sftp_sensor.py	100% <100%> (ø)
airflow/providers/sftp/operators/sftp_operator.py	87.67% <87.67%> (ø)
airflow/providers/sftp/hooks/sftp_hook.py	94.84% <94.84%> (ø)

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[TobKed]

cc @mik-laj

[mik-laj]

We currently have open discussions to move integration to other package.

CC: @potiuk

[TobKed]

@potiuk @mik-laj I've moved SFTP hook, operator and sensor to providers/sftp folder.
I've included __init__.py files due to the problem with from tests.test_utils.config import conf_vars (without __init__ files import was not possible)

[TobKed]

CC @mik-laj @potiuk