Identify code that needs deprecation and removal for custom file extension removal [#3627210]

Problem/Motivation

.theme, .module, and soon .profile file extensions have been deprecated. There is a lot of code supporting these, we should identify what code can be dropped once we remove support.

Some is called so often we can't truly deprecate it.

Attributes

LegacyHook
ProceduralCall
LegacyRequirementsHook
ProceduralHookScanStop
ExtensionFileIsConverted

Container parameter

skip_procedural_hook_scan

HookCollectorBase

$procedural_files

HookCollectorPass and ThemeHookCollectorPass

$proceduralFileExtensions
$proceduralImplementations
$procedural_files
checkForProceduralOnlyHooks
addProceduralImplementation
$staticDenyHooks
getThemeListPattern
getModuleListPattern
$deprecatedThemeFiles

ModuleHandler

$loadedFiles
$loaded
load
loadAll
reload
isLoaded
legacyInvoke

ThemeInitialization

loadActiveTheme

Extension

$filename
getExtensionPathname
getExtensionFilename
load

ThemeRegistry is loaded
Registry is loaded

Tests that need a closer look:
core/modules/user/tests/src/Kernel/UserMailNotifyTest.php
Drupal\Tests\help\Functional\HelpTopicTranslation
Drupal\Tests\package_manager\Kernel\SupportedReleaseValidator
Schema loading in kernel tests

public static function getTablesSpecification(ModuleHandlerInterface $handler, string $module): Schema|array {
if ($handler->loadInclude($module, 'install')) {
$schema = $handler->invoke($module, 'schema') ?? [];

# Delete folders

core/modules/system/tests/modules/hook_collector_on_behalf_procedural
core/modules/system/tests/modules/legacy_hook_test
core/modules/system/tests/modules/hook_collector_skip_procedural
core/modules/system/tests/modules/module_test_procedural_preprocess
core/modules/system/tests/modules/container_initialize
core/modules/system/tests/modules/hook_collector_skip_procedural_attribute
core/tests/Drupal/Tests/core/Extension/modules/module_handler_test_all2
core/tests/Drupal/Tests/core/Extension/modules/module_handler_test_all1
core/tests/Drupal/Tests/core/Extension/modules/module_handler_test_added

# Delete files

core/modules/editor/editor.admin.inc
core/modules/field/field.purge.inc
core/modules/locale/locale.batch.inc
core/modules/locale/locale.compare.inc
core/modules/locale/locale.fetch.inc
core/modules/locale/locale.bulk.inc
core/modules/locale/locale.translation.inc
core/modules/views_ui/admin.inc
core/modules/update/update.compare.inc
core/tests/Drupal/Tests/core/Extension/modules/module_handler_test/module_handler_test.module
core/modules/system/tests/modules/module_autoload_test/module_autoload_test.module
core/modules/system/tests/modules/HookOrder/aaa_hook_order_test/aaa_hook_order_test.module
core/modules/system/tests/modules/HookOrder/ccc_hook_order_test/ccc_hook_order_test.module
core/modules/system/tests/modules/HookOrder/ddd_hook_order_test/ddd_hook_order_test.module
core/modules/system/tests/modules/HookOrder/bbb_hook_order_test/bbb_hook_order_test.module
core/modules/menu_ui/menu_ui.module
core/modules/user/user.module
core/modules/editor/editor.module
core/modules/taxonomy/taxonomy.module
core/modules/block/block.module
core/modules/text/text.module
core/modules/system/system.module
core/modules/image/image.module
core/modules/filter/filter.module
core/modules/field/field.module
core/modules/jsonapi/jsonapi.module
core/modules/dblog/dblog.module
core/modules/locale/locale.module
core/modules/comment/comment.module
core/modules/language/language.module
core/modules/node/node.module
core/modules/views/views.module
core/modules/content_translation/content_translation.module
core/modules/syslog/syslog.module
core/modules/file/file.module
core/modules/options/options.module
core/modules/update/update.module
core/modules/automated_cron/automated_cron.module
core/profiles/tests/testing_hooks/testing_hooks.profile