Problem/Motivation
It looks like we broke a fair number of contributed modules by introducing the \Drupal\Core\Form\ConfigFormBase::$typedConfigManager property in #3364506: Add optional validation constraint support to ConfigFormBase because some contributed modules already have this property in their config forms but without the typehint. However, it looks like some modules have it without the typehint and some with.
Proposed resolution
Should we try changing the property name to something that is less likely to conflict with the base class? We could also trigger deprecation if <code>\Drupal\Core\Form\ConfigFormBase::$typedConfigManager exists so that contrib knows to remove the duplicate properties.
