Since the DownloaderService's events are all based on the complete download
URLs, and RepoUpdater is where the update URLs are built, this makes the
full download URL into a read-only property of RepoUpdater so it can be
used wherever there is an instance of RepoUpdater
This is also important because having the `final` property highlights
the lifecycle of that variable: it does not change during the entire
life of a RepoUpdater Instance.
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