This is continuation of the separation of the Airflow codebase into
separate distributions. This one splits airflow into two of them:

* apache-airflow - becomes an empty, meta no-code distribution that
  only has dependencies to apache-airflow-core and task-sdk
  distributions and it has preinstalled provider distributions
  added in standard "wheel" distribution. All "extras" lead
  either to "apache-airflow-core" extras or to providers - the
  dependencies and optional dependencies are calculated differently
  depending on "editable" or "standard" mode - in editable mode,
  just provider dependencies are installed for preinstalled providers
  in standard mode - those preinstalled providers are dependencies.

* the apache-airflow-core distribution contains all airflow core
  sources (previously in apache-airflow) and it has no provider
  extras. Thanks to that apache-airflow distribution does not
  have any dynamically calculated dependencies.

* the apache-airflow-core distribution hs "hatch_build_airflow_core.py"
  build hooks that add custom build target and implement custom
  cleanup in order to implement compiling assets as part of the build.

* During the move, the following changes were applied for consistency:

  * packages when used in context of distribution packages have been
    renamed to "distributions" - including all documentations and
    commands in breeze to void confusion with import packages
    (see
https://packaging.python.org/en/latest/discussions/distribution-package-vs-import-package/)

  * all tests in `airflow-core` follow now the same convention
    where tests are in `unit`, `system` and `integration` package.
    no extra package has been as second level, because all the
    provider tests have "<PROVIDER>" there, so we just have to avoid
    naming airflow unit."<PROVIDER>" with the same name as provider.

  * all tooling in CI/DEV have been updated to follow the new
    structure. We should always build to packages now when we
    are building them using `breeze`.