#4 do_download only proceed on url file exists, also:

updgrade(): change so only upgrades named packages, or "all; doc updates. Note: UNTESTED because we have to address rest of #4 first.
#4: change default mirror; #5 check for archive first...

Not tested. We require a situation where:

- a new version for an installed package is available, and
- the package has dependencies which are not installed

This speaks to the need for a real test suite (#6 here we come!)