depends: add a fallback path in case package sources go missing

If a source url fails to download, try again at
$FALLBACK_DOWNLOAD_PATH/file.name, where $FALLBACK_DOWNLOAD_PATH can be
overridden by the user. By default it's https://bitcoincore.org/depends-sources

Of course, this depends on https://bitcoincore.org/depends-sources actually being populated with these sources before it's useful...