Rename PackIndex->PackIdx and MultiPackIndex->PackIdxList.

Rename PackIndex->PackIdx and MultiPackIndex->PackIdxList.

This corresponds to the PackMidx renaming I did earlier, and helps avoid
confusion between index.py (which talks to the 'bupindex' file and has
nothing to do with packs) and git.py (which talks to packs and has nothing
to do with the bupindex).  Now pack indexes are always called Idx, and the
bupindex is always Index.

Furthermore, MultiPackIndex could easily be assumed to be the same thing as
a Midx, which it isn't.  PackIdxList is a more accurate description of what
it is: a list of pack indexes.  A Midx is an index of a list of packs.