Don't create more than one notebook detection task per-type (microsoft#167878)

Don't create more than one notebook detection task per-type.
This is creating detection tasks to cover the period between opening a notebook, and the notebook extension activating and starting its own detection task. This code is a bit more complex than necessary, and it would be simpler to manage the lifecycle of this task from the `activateByEvent` promise.
Fix microsoft#167875

Author: roblourens

src/vs/workbench/contrib/notebook/browser/contrib/kernelDetection/notebookKernelDetection.ts

@@ -58,7 +58,7 @@ class NotebookKernelDetection extends Disposable implements IWorkbenchContributi
 						}
 					});
 
-					if (shouldStartDetection) {
+					if (shouldStartDetection && !this._detectionMap.has(notebookType)) {
 						const task = this._notebookKernelService.registerNotebookKernelDetectionTask({
 							notebookType: notebookType
 						});