..which is created (touched) *after* successful download of all tarballs.
fetch.log is not a good target because we touch it along the way.

Note: This is necessary only because we write pointless? date into fetch.log.
Otherwise 'download' could be phony target, thanks to the rule being empty
if we don't need to download anything.

Change-Id: I2380ab189833e03758ddd9f9bf54be38c8fa1dbe