cleanup upgrade API some more (thanks for the feedback from David)
re-add "Calculating upgrade..." message
* move upgrade releated code into upgrade.{cc,h}

The upgrade releated code is moved into upgrade.{cc,h} and
all pkg*Upgrade* prototypes are included in algorihms.h to
avoid breaking API (unless build with APT_9_CLEANER_HEADERS).
Move ListUpdate/AquireUpdate into update.{cc,h}

This moves the ListUpdate/AquireUpdate out of the "catch-all"
algorithm.{cc,h} file into its own update.{cc,h}