Issue #2917883 by quietone, TR, Jo Fitzgerald, masipila, phenaproxima: Rename 'migration_templates' directories in core modules to 'migrations'

Revert "Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal"

This reverts commit 89815ba3.

Issue #2560795 by mikeryan, phenaproxima, penyaskito, willwh, benjy: Source plugins have a hidden dependency on migrate_drupal