DfsPackFileMidx: Introduce single-pack midx

We expect to have midxs covering a single pack (GC) in the midx
chain. With the current implementation, this would load twice the same
information, in the midx and in the pack indexes.

Create a new subclass for the single-pack case.

Change-Id: I9de2a282d812e0078c9f1c1ae6f3f40d6a6a6964

Author: ifradeo