MultiPackIndex: use .midx files if they exist.

MultiPackIndex: use .midx files if they exist.

Wow, using a single .midx file that merges my 435 megs of packfile indexes
(across 169 files) reduces memory churn in memtest.py by at least two orders
of magnitude.  (ie. we need to map 100x fewer memory pages in order to
search for each nonexistent object when creating a new backup)  memtest.py
runs *visibly* faster.

We can also remove the PackBitmap code now, since it's not nearly as good as
the PackMidx stuff and is now an unnecessary layer of indirection.