PARAVIEW_EXTERNAL_PLUGIN_DIRS requires plugin directory to contains a plugin.cmake
The PARAVIEW_EXTERNAL_PLUGIN_DIRS cmake variable enable user to point external plugin directories. However, these directories must contain plugin.cmake files in order for the plugin to be built. If not, no warning or error or produced.
It would be nice that if no plugin.cmake file are present in the file, the CMakeLists.txt would still be parsed and the plugin built.
As a side note, Example Plugins in ParaView do not contains any plugin.cmake file.