only process if there is an update

[todo]

swift-package-registry/lib/add.js

Lines 59 to 64 in e685aa0

	// todo: only process if there is an update
	processingQueue.push(_package)
	}
	if (processingQueue.length <= 1) {
	parsePackageQueue(processingQueue)

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This issue was generated by todo based on a todo comment in e685aa0. It's been assigned to @twodayslate because they committed the code.

[twodayslate]

When adding a package a conditional request (see #13) should be used to see if there is actually an update to this project. If there is an update then add again to get the latest version