23 files declare their use of ExtensionDiscovery in Drupal 9

BaseThemeMissingTest.php in core/tests/Drupal/KernelTests/Core/Theme/BaseThemeMissingTest.php

Database.php in core/lib/Drupal/Core/Database/Database.php

DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php

EntityFilteringThemeTest.php in core/modules/system/tests/src/Functional/Theme/EntityFilteringThemeTest.php

ExtensionDiscoveryTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php

ExtensionInstallStorage.php in core/lib/Drupal/Core/Config/ExtensionInstallStorage.php

ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php

FieldTypePluginManagerTest.php in core/modules/field/tests/src/Kernel/FieldTypePluginManagerTest.php

GenerateTheme.php in core/lib/Drupal/Core/Command/GenerateTheme.php

install.core.inc in core/includes/install.core.inc

API functions for installing Drupal.

install.inc in core/includes/install.inc

API functions for installing modules and themes.

InstallCommand.php in core/lib/Drupal/Core/Command/InstallCommand.php

InstallStorage.php in core/lib/Drupal/Core/Config/InstallStorage.php

KernelTestBase.php in core/tests/Drupal/KernelTests/KernelTestBase.php

module.inc in core/includes/module.inc

API for loading and interacting with Drupal modules.

SystemListingTest.php in core/modules/system/tests/src/Kernel/Common/SystemListingTest.php

TestDiscovery.php in core/lib/Drupal/Core/Test/TestDiscovery.php

TestInstallStorage.php in core/modules/config/tests/config_test/src/TestInstallStorage.php

TestRequirementsTrait.php in core/tests/Drupal/Tests/TestRequirementsTrait.php

TestRunnerKernel.php in core/lib/Drupal/Core/Test/TestRunnerKernel.php

ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php

ThemeSettingsTest.php in core/tests/Drupal/KernelTests/Core/Theme/ThemeSettingsTest.php

UpdateRegistry.php in core/lib/Drupal/Core/Update/UpdateRegistry.php