Fix three bugs with fake-map implementation for PackageIndex.

1. When we union PackageIndexes together, prefer the later one.
   This idiom is used when we update the processing-state of
   packages in an InstallPlan.

2. dependencyInconsistencies' was missing a number of indirections
   through the fakeMap, so in some cases we incorrectly concluded
   packages were not equal when they were.

3. We need to initialize the fakeMap with any pre-installed packages,
   otherwise the invariant check for configured-packages will fail.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>