Enable back iterative development of latest providers with old airflows

[potiuk]

The compatibility tests in CI are using providers built as packages from sources, so the compatibility tests run there using "providers/tests" work just fine, because all providers are installed in the airflow.providers site library. However when we are iterating and debugging backwards compatiblity provider tests, we should be able to use local provider sources, rather than installed packages and we have the possibility of mounting both - providers sources and tests to the image.

See contributing-docs/testing/unit_tests.rst on how to do it by using --mount-sources providers-and-tests flag connected with --use-airflow-version.

However as of #42505 this has been broken, because currently in main we rely on airflow having "pkgutil" namespace package for both - airflow, and airflow.providers packages (previous airflow versions had implicit package for airflow.providers package) - so providers installed locally cannot be used as "another" source of providers. Previously it was working because both "installed" and "sources" airflow.providers package were implicit namespace packages.

As explained in https://packaging.python.org/en/latest/guides/packaging-namespace-packages/#native-namespace-packages

Every distribution that uses the namespace package must include such
an __init__.py. If any distribution does not, it will cause the
namespace logic to fail and the other sub-packages will not be
importable. Any additional code in init.py will be inaccessible.

So because old airflow uses implicit provider's packages and main airflow from source uses "explicit" provider's package, the only way we can make the "source" providers is to mount them or symbolically link them to inside installed distribution of airflow package (in site directory) (or dynamically remove the init.py from provider's source directory.

We cannot mount the provider package sources ot inside the installed airflow - because when --use-airflow-version is used, airflow is installed dynamically inside the container - after the container is started.

This PR solves the problem by adding an env variable that will make the initialization script to remove the installed airflow.providers folder after installing airflow and linking the "providers/src/airflow/providers" folder there. This has the added benefit that all providers (including the preinstalled ones) are used from "main" sources rather than from installed packages - which was problematic for the past way of using providers from sources - which used the fact that both "airflow.providers" in the site-library and the one in sources were implicit namespace packages.

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[potiuk]

Very interesting :). I've learned a bit more about namespace packages.

[potiuk]

Found it while working on #43556

[gopidesupavan]

Definitely this is a great catch @potiuk :) Thank you..