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 #2625696 by chx, benjy, alexpott, catch, mikeryan, andypost, phenaproxima: Make migrations themselves plugins instead of config entities

Issue #2534004 by phenaproxima, mikeryan: Move module-specific migration support into the comment module

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 #2314289 by benjy: Track result of migrations and use it to properly define migrate_dependencies.

Issue #2293773 by Gábor Hojtsy, alexpott, penyaskito: Fixed migrations use dots in key names - not allowed in config.

Issue #2228763 by larowlan, alexpott, martin107, jessebeach, xjm: Create a comment-type config entity and use that as comment bundles, require selection in field settings form.

Issue #2121299 by andypost, bdone, benjy, penyaskito, chx, claudiu.cristea, damiankloip, gregboggs, InternetDevels, jessehs, jhedstrom, marvil07, mikeryan, pcambra, Xano, YesCT, axoplasm, kgoel, dclavain, jeckman, alvar0hurtad0, Drupali, fastangel, gloob, jhedstrom, joshtaylor, mpgeek, alexpott: Migrate Drupal 6 to Drupal 8