uscan: apply uversionmangle rules to --download-version.

Closes: #1087641 — devscripts: uscan --download-version fails to match upstream versions when uversionmangle rules are used

Currently uscan compares the version supplied by the user as the value of --download-version command line argument, usually the actual version as it appears on upstream tags and packages, e.g. 0.12-workingonit-34, to the umangled version computed from links on a web page or API request reply, which is usually mangled to something like 0.12~workingonit-34 or possibly something completely different e.g. if the 0.12-workingonit-* releases are actually anterior to the 0.12-almostdone-* releases and the umangle rules were tweaked to sort them correctly as suggested by the documentation. As a result the requested version does not match what is computed and the download fails.

It seems to me that it would make sense to also always apply the same umangle rules to the --download-version supplied by the user so they can always use upstream versions as they appear.