private void preprocessFolder(DomainFolder folder, TaskMonitor monitor)
			throws CancelledException {
		if (foldersToDelete.containsKey(folder)) {
			return;
		}

		monitor.setMessage("Checking " + folder.getPathname());
		foldersToDelete.put(folder, null);
		for (DomainFile file : folder.getFiles()) {
			monitor.checkCancelled();
			preprocessFile(file, monitor);
			monitor.incrementProgress(1);
		}

		for (DomainFolder subFolder : folder.getFolders()) {
			monitor.checkCancelled();
			preprocessFolder(subFolder, monitor);
		}
	}
Iterates over the user-supplied files and folders, saving the discovered files and
subfolders and their files into {@link #filesToDelete} and {@link #foldersToDelete}.

@param monitor
@return
Ghidra/Framework/Project/src/main/java/ghidra/framework/main/projectdata/actions/ProjectDataDeleteTask.java