Raise helpful error when using the wrong plugin base classes (#8893)

Author: jacobtomlinson

distributed/client.py

@@ -5101,6 +5101,14 @@ def _register_plugin(
     ):
         if isinstance(plugin, type):
             raise TypeError("Please provide an instance of a plugin, not a type.")
+        if any(
+            "dask.distributed.diagnostics.plugin" in str(c)
+            for c in plugin.__class__.__bases__
+        ):
+            raise TypeError(
+                "Importing plugin base classes from `dask.distributed.diagnostics.plugin` is not supported. "
+                "Please import directly from `distributed.diagnostics.plugin` instead."
+            )
         raise TypeError(
             "Registering duck-typed plugins is not allowed. Please inherit from "
             "NannyPlugin, WorkerPlugin, or SchedulerPlugin to create a plugin."