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

Issue #2423103 by phenaproxima, quietone, miguelc303, mikeryan: Migration Files for Drupal 7 Content

Issue #2463909 by phenaproxima, mikeryan: Migrations should support non-installed default configurations (templates)

Issue #2460529 by alexpott, chx, phenaproxima: Migrations need to use the configuration entity dependency system

Issue #2466797 by mikeryan, benjy: Rename migration_groups key to avoid confusion with MigrationGroup support

Issue #2358269 by Gábor Hojtsy: Migration bugs in block visibility, field overrides, cron, maintenance settings and form modes found by configuration schema checking