Services from core contrib modules missing!!!! "The service xxxx has a dependency on a non-existent service xxx. " (with solution)
A contractor had added a directory called "webform" to modules/custom to add some custom configurations to webform. Which worked fine after updating webform module, until you cleared cache. Then Drupal blew up with the above error (similar from drush side) and I got puzzled silence on the call with them.

Turns out Drupal looks in custom first, for module directories, and ignores directories of the same name in contrib. When other stuff tried to reference services in the contrib module, drupal stopped looking for stuff in "webform" at the custom directory of the same name and died.

Hopefully this saves someone some time. Learn from our folly!

Changing the name custom/webform to custom/SITE_NAME_webform (with .info.yml etc name changes) fixed issue.

They're killing me.