Remove packIndex field from FileObjDatabase openPack method.

Remove packIndex field from FileObjDatabase openPack method.

Previously, the FileObjDatabase required both the pack file path and
index file path to be passed to openPack().  A future change to add
a bitmap index will add a .bitmap file parallel to the pack file
(similar to the .idx file). Update the PackFile to support
automatically loading pack index extensions based on the pack file
path.

Change-Id: Ifc8fc3e57f4afa177ba5a88df87334dbfa799f01