refs #19321

Thanks carrie.hazelton at altamiracorp.com for the report.

Thanks alex_koval for the report.

Thanks Tim for the review.

Typo fix for exception message.

GEOS 3.3 added crosses, disjoint, overlaps, touches and within
predicates on prepared geometries.

Thanks Robert Scott for the report.

Also renamed app_cache to apps and "app cache" to "app registry".

Deprecated AppCache.app_cache_ready() in favor of Apps.ready().

Invalidate properly the cache whenever all_models or app_configs change.
This fixes some isolation issues in the test suite.

Previously, this would corrupt the settings, because __exit__ isn't
called when __enter__raises an exception.

It's now easier to achieve the same effect with modify_settings or
override_settings.

* Introduced [un]set_installed_apps to handle changes to the
  INSTALLED_APPS setting.
* Refactored [un]set_available_apps to share its implementation
  with [un]set_installed_apps.
* Implemented a receiver to clear some app-related caches.
* Removed test_missing_app as it is basically impossible to reproduce
  this situation with public methods of the new app cache.

It was not used inside Django, is not tested or documented. Consequently
remove without deprecation path.

Thanks to @vajrasky for bringing it to our attention.

That matches its return type better.

That matches its name ad its purpose better.

It is a public API.

Reverted 4a56a93c.

Currently such overrides aren't reflected in the app cache.

It would  be possible to handle them. But that doesn't look like a very
good API. It makes it complicated to express "add this app" and "remove
this app", which are the most common operations on INSTALLED_APPS.

- Tested consistency the current app_configs instead of INSTALLED_APPS.
- Considered applications added with _with_app as available.
- Added docstrings.

Except the app cache code and a few specific tests, of course.

Removed some exception masking in the comments app that was harmful and
couldn't be preserved easily.