is_valid_plugin can no longer be used for unit testing plugins

[soggycactus]

I can no longer use airflow.plugins_manager.is_valid_plugin to write a simple unit test that ensures I can't merge broken plugins to master.

This is because the function no longer accepts a list of plugins that represents existing plugins, and instead references a global variable inside the package (I would really love an explanation as to how this is better).

The global variable plugins is defined as None, and when calling the function locally, it will always fail because a None object is not iterable.

Either this function needs to be fixed or Airflow needs to provide another interface for validating plugins. I was really disappointed to see this as I spent this week researching how to upgrade our Airflow to 2.0, only to find that a lot of the utilities I've built around Airflow have been rendered useless. I'm sticking with 1.10.14 for now

[boring-cyborg]

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[mik-laj]

@soggycactus This feature doesn't really do much work, so I don't think it's too useful for the end-user. From my experience, I know that it is best to check such things from the user's perspective, because this is the way to detect most problems.
This means that you can run the code below to check if your plugin loads correctly.

with mock.patch.dict('os.environ',  AIRFLOW__CORE__PLUGINS_FOLDER='/tmp/plugins_dir'):
    assert 'metastore_browser' in subprocess.check_output(['airflow', 'plugins'], timeout=15):