Resolve #3506605 "Config entity source"

Closes #3506605

added 5 commits

• 97e26340 - Copy the test from migrate_drupal to migrate
• ce0ef761 - Update the test for the migrate module
• cb368798 - Mark the existing test as legacy
• c4ff2654 - Remove unused use statements
• fd346360 - Fix the constructor with the deprecation message

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added 1 commit

• 9229f69e - Add an @see comment after the @deprecated comment

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added 1 commit

• e0ff54c5 - Tell the automated tests to expect the new source plugin

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closed