Removed the unused provider's package

[potiuk]

This is a set of cleanup steps (first stage) that allow us to remove
the "intermediate" provider's distribution from Airlfow code and replace
it fully with individual provider's distributions - already with own
pyproject.toml files and basically being (when we complete) a
completely separate distributions from Airflow and without implicit
dependencies between unrelated distributions.

There are a number of other changes needed but that one is only focusing
on removing all references to the "umbrella" providers distribution
and consequences of removing it.

Those are the changes implemented in this PR:

• There are no separate "providers" system tests - each provider has
  own system tests and there are no common "generic" providers empty
  system test

• Integration tests are moved to respective providers under the
  integration package inside tests directory

• (nearly) empty init.py files are added in tests directories
  of providers - this way "tests" becomes just a directory and root
  for all tests per provider, rather than a Python package on its own.
  That allows to use "from integration.PROVIDER import" and
  "from system.PROVIDER" rather than importing them from the root of
  the whole airflow project. The (nearly) is because we need to
  handle multiple "system", "system.apache" and other import locations.

• Removed references to "providers/" generic package which were
  scheduled for removal after all providers are moved to the new
  structure

• Few remaining references / links referring to old "providers/src" and
  "providers/test" have been fixed.

• The "conftest.py" files in all providers are trimmed down - the code
  to store ignored deprecation warnings have been moved to the
  test_common pytest_plugin. That allows to remove 90+ duplicated
  snippets of deprecation_warnings retrieval while keeping the warnings
  per-provider in the provider's distribution.

• The "moving_providers" scripts are removed. They've done their job and
  are not needed any more - we keep them in history

• The www tests that were using FAB permisssion model are moved to the
  FAB provider tests.

---

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[o-nikolas]

The existence of this directory for the amazon provider was one of the sneaky ways in which tests were breaking when I was migrating that provider. Amazon is used as an example in a lot of tests and some code was detecting it as not migrated because of that.

Any who, I made a note to come back this morning and delete the others but you've already got it, nice!

[potiuk]

There are still few errors due to cross-provider dependencies with fab provider / azure paths not fixed - but generally it should be ready for review (I will fix the errors tomorrow).

[potiuk]

For those who will be reviewing this one - I recommend git diff -> UI of Github is very, very slow with that huge of a change.

[jscheffl]

Just a few sanity check comments, compared to Niko I did not find any glitches. Good to be merged after nit's applied.

[potiuk]

Just a few sanity check comments, compared to Niko I did not find any glitches. Good to be merged after nit's applied.

👀 👀 👀 👀 👀 👀 👀 👀

[potiuk]

Rebased and applied the nits.

[potiuk]

BTW.I plan to have a separate discussion on the "final" layout of the provider test code so I plan to merge it rather quickly and then write up a discussion thread where I would want people to chime in and make some final decisions. I am afraid if I won't merge it quickly some test conflicts will take more than they are worth. So @o-nikolas @Lee-W -> I think, if the tests pass after rebase, I will merge it, but it is really up to a follow up discussion how the folder and structure and imports will look like eventually (this is all test code so we can remove it again it will be far simpler to do follow-up renames.

[potiuk]

Merged !