Add ImportChangesRule for upgrade check

[turbaszek]

Adds new rule that will be run during airflow upgrade-check
and will look for old clasess and imports incompatible with
Airflow 2.0.

Example:

Changes in import paths of hooks, operators, sensors and others
---------------------------------------------------------------
Many hooks, operators and other classes has been renamed and moved. Those changes were part of unifying names and imports paths as described in AIP-21.
The `contrib` folder has been replaced by providers directory and packages:
https://github.com/apache/airflow#backport-packages

Problems:
  1.  Using `airflow.contrib.sensors.python_sensor.PythonSensor` will be replaced by `airflow.sensors.python.PythonSensor`. Affected file: /files/dags/test_dag_1.py
  2.  Using `airflow.operators.bash_operator.BashOperator` will be replaced by `airflow.operators.bash.BashOperator`. Affected file: /files/dags/xyz.py
  3.  Using `airflow.operators.bash_operator.BashOperator` will be replaced by `airflow.operators.bash.BashOperator`. Affected file: /files/dags/generate_dags.py

Related to #8765

---

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Read the Pull Request Guidelines for more information.
In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.

[turbaszek]

cc @vikramkoka

[turbaszek]

The docs faliure was already fixed.

[turbaszek]

@olchas @mik-laj @TobKed can you please take a look?

[kaxil]

can be simplified

Suggested change

	class ImportChange(
	NamedTuple(
	"ImportChange",
	[("old_path", str), ("new_path", str), ("providers_package", Optional[None])],
	)
	):
	class ImportChange(NamedTuple)
	old_path: str
	new_path: str
	providers_package: Optional[None]

[kaxil]

oh wait, I know why you did this, for compatibility with Py 2.7 , Py 3.5 ?

[turbaszek]

Yup, not a big fan of this but we this command has to work with 2.7

[JeffryMAC]

Some operators did the other way.. from providers to core
#10013
this message will be wrong for it?

[turbaszek]

The message will be:

Using airflow.operators.email_operator.EmailOperator will be replaced by airflow.operators.email.EmailOperator.

And I see no place for confusion here