Use fresh WebClient for fallback URLs

[HebaruSan]

Problem

Sometimes when downloading mods, CKAN indicates negative bytes remaining and greater than 100% progress towards completion. This problem has been reported on both CmdLine and GUI, and presumably ConsoleUI would be affected as well:

Cause

When the primary download URL for a redistributable mod fails, we fall back to its archive.org URL as of #2284. This is done by calling agent.DownloadFileAsync in the agent.DownloadFileCompleted event handler when it indicates failure. Notably this re-uses the same WebClient object.

Since the negative bytes remaining and greater than 100% progress numbers come directly from values provided by WebClient, I think the WebClient object is getting corrupted by this sequence of events. It may not be prepared for us to initiate a new download while the previous one is still being dealt with.

Changes

NetAsyncDownloaderDownloadPart'sWebClient object is now changed from public to private. Small wrapper functions and events are added to handle the operations that this object performs, and NetAsyncDownloader is updated to call them:

• Download - Replaces agent.DownloadFileAsync
• Abort - Replaces agent.CancelAsync
• Progress - Replaces agent.DownloadProgressChanged
• Done - Replaces agent.DownloadFileCompleted

This encapsulation allows NetAsyncDownloaderDownloadPart.Download to ensure that its WebClient object is replaced and re-initialized before any download operation. Rather than re-using a potentially scrambled object, fallback URLs will now have their own fresh WebClient.

Should fix #2508, but I can't make this problem happen on demand so testing is inconclusive.