Fixed regression in 4dc4d12e. Refs #21598.

That commit contained a mistake that resulted in the use_cached_loader
option of override_with_test_loader being ignored. As a consequence some
configurations weren't exercised any more by the test suite.
Moved all template loaders under django.template.loaders.

Reformatted the code of base.Loader according to modern standards.

Turned the test template loader into a regular locmem.Loader -- but
didn't document it.

Added a normal deprecation path for BaseLoader which is a public API.

Added an accelerated deprecation path for TestTemplateLoader which is
a private API.
Removed the "test:" prefix from locmem template identifiers.

Since it isn't branded as a test utility any more and could be used for
other purposes than test code, that prefix no longer makes sense.

It wasn't used anywhere either.
Removed skip_template argument of locmem.Loader.load_template_source.

It didn't do anything, wasn't documented and wasn't used anywhere.
Refactored listing template subdirectories in apps.

This change has the nice side effect of removing code that ran at import
time and depended on the app registry at module level -- a notorious
cause of AppRegistryNotReady exceptions.
Removed override_template_loaders and override_with_test_loader.

They can be replaced with override_settings and that makes the
corresponding tests much more obvious.
Removed obsolete comment.

It didn't account for class-based template loaders.
Deprecated function-based loaders.
Refactored getting the list of template loaders.

This provides the opportunity to move utility functions specific to the
Django Template Language outside of django.template.loader.
Used get_template_loaders in the cached loader.

This ensures that enabling the cached loader doesn't change behavior.

(Before this commit, it did when the list contained unusable loaders.)