Merged from trunk 5188:5196

Merged from trunk 5188:5196

Use a fake vartree for depgraph calculations.  This minimizes the time
that a lock needs to be held on the vardb, and allows in-memory global
updates when necessary.

Delete the depgraph when it's no longer needed.

Use FakeVartree for vardb lock/cache during depclean.

Allow depclean to silently skip packages unmerged behind it's back.

Fix depclean so the fake vartree is used in dep_check calls.

Always silently ingnore missing packages during unmerge.

Remove all traces of the unmerge raise_on_missing parameter.

Fix indentation.

svn path=/main/branches/prefix/; revision=5223