Unresolved reference to qgis.core

I am updating a set of proprietary plugins from Python 2.7/QGIS 2.2 to Python 3.7/QGIS version 3.6.1-Noosa and am unable to reconcile the reference to qgis.core. It is installed here: C:\Program Files\QGIS 3.6\apps\qgis\python\qgis and it is included in the PATH variable.