Make depgraph creation more tolerant of missing or masked packages when

Make depgraph creation more tolerant of missing or masked packages when
the relevant deps are satisfied by installed packages. This kind of
friendliness is especially desired in cases such as --emptytree where
it might not be possible to reinstall every single package. Also, it
allows multislot atoms from the world file (that are necessary to prevent
them from being removed by depclean) trigger warning messages while
still allowing a --emptytree to proceed.

svn path=/main/trunk/; revision=9645