Remove Migrate Drupal

Problem/Motivation

The removal of extension Migrate Drupal was approved in #3371229: [Policy] Migrate Drupal and Migrate Drupal UI after Drupal 7 EOL.

Migrate Drupal is not being moved to contrib.

Remaining tasks

1. ✅ Remove most of the extension. Keep only the info file and uninstall hook so sites that have it around can uninstall it. Mark it lifecycle: obsolete so new sites don't install it. Add a hook_update_N() to uninstall the module.
2. ✅ Remove references from core/phpstan-baseline.neon.
3. ✅ Spelling words have not been removed. This is better done in a dictionary rebuild before 12.0 beta, I guess.
4. ✅ Removed exclusion of migrate drumps from cspell.,json
5. ✅ Add the extension to the relevant removed list, either DRUPAL_CORE_REMOVED_MODULE_LIST or DRUPAL_CORE_REMOVED_THEME_LIST, in system.install.
6. Check for references in @todo.

✅ core/modules/migrate/tests/src/Functional/MigrateNoMigrateDrupalTest.php - this was to test a hidden dependency on migrate drupal in the migrate module. So, this can be deleted.

✅ core/modules/migrate/tests/src/Kernel/Plugin/MigrationPluginConfigurationTest.php. - changed the test to not rely on the locale module and the legacy source plugin, Variable.

✅ Add both migrate_drupal and migrate_drupal_ui to DRUPAL_CORE_REMOVED_MODULE_LIST. The latter was missed in #3574714: Remove Migrate Drupal UI from core

Key things to Review

The changed help text in the Migrate module.
MigrationPluginConfigurationTest.php.

Commands used

Listing here the various bash commands I used to remove Migrate Drupal. Each command was run by hand, I never put it all into a script. These removed the bulk of the files.

# MigrateField Plugins
git grep -l '\#\[MigrateField' | xargs rm

# List of plugins taken from the change records
cat plugin-remove.txt | xargs rm

# The migration yml
find . -depth -type d -name migrations -not -path '*/migrate/*' -not -path '*/migrate_drupal_ui/*'  -exec rm -rf {} \;

# Remove migrate 'source' directories
find . -depth -type d -name source -not -path '*/core/modules/migrate/*' -not -path '*/migrate_drupal/*' -not -path '*/migrate_drupal_ui/*' -not -path '*/node_modules/*' -exec rm -rf {} \;

# Remove Kernel tests
find core -depth -type d -name Migrate -not -path '*/core/modules/migrate*' -not -path '*/migrate_drupal*' -not -path '*/migrate_drupal_ui*' -not -path '*/node_modules/*' -exec rm -rf {} \;

# Remove d6 and d7 directories
find core -depth -type d -name d6 -not -path 'core/modules/migrate*' -not -path '*/migrate_drupal*' -not -path 'core/node_modules/*' -exec rm -rf {} \;
find core -depth -type d -name d7 -not -path 'core/modules/migrate*' -not -path '*/migrate_drupal*' -not -path 'core/node_modules/*' -exec rm -rf {} \;

# Remove files starting with 'D6' or 'D7'.
find . -name D6\*.php -exec rm {} \;
find . -name D7\*.php -exec rm {} \;

# Remove more tests
find core/modules/*/tests -depth -type d -name migrate -not -path 'core/modules/migrate*' -not -path '*/migrate_drupal*' -not -path 'core/node_modules/*' -not -path 'core/modules/menu_link_content/*' -exec rm -rf {} \;

# Remove field module process plugins
find core/modules/field -depth -type d -name migrate -exec rm -rf {} \;

Release notes snippet

The Migrate Drupal module is removed.
The Migrate Drupal module was deprecated in Drupal 11.4.0. There is no replacement contributed module.