Improve lookahead for new-style virtuals:

Improve lookahead for new-style virtuals:

* Substitute the graph tree for the vartree in dep_check() since we
  want atom selections to be consistent with package selections
  that have already been made.
* Add the selected package to the graph as soon as possible
  so that later dep_check() calls can use it as feedback
  for making more consistent atom selections.
* Expand all slots of new-style virtuals inside _dep_check_composite_db
  so that lookahead examines all matched slots.

This solves some cases of bug #1343, and extends the fix for bug
#141118 to work in cases when a virtual is not yet installed but
it has been pulled into the graph.

(trunk r9873)

svn path=/main/branches/2.1.2/; revision=9877