In depgraph.create(), don't ignore direct circular dependencies

In depgraph.create(), don't ignore direct circular dependencies
anymore since altlist() is able to handle it properly in cases
where the dependency is satisfied. If the dep is unsatisfied
then it can not be ignored. (trunk r8225)

Allow the depgraph to add old-style virtual providers but prefer
any pre-existing providers over new ones that are added. This
prevents things like ghc-6.4.2 from selecting itself to satisfy
it's own virtual/ghc dependency (creating a circular dependency)
in cases where it should select the default ghc-bin provider.
(trunk r8185:8187)

svn path=/main/branches/2.1.2/; revision=8229