PackIndex: Simplify Iterator/MutableEntry interaction

PackIndex: Simplify Iterator/MutableEntry interaction

The iterator keeps the current position in the index and the MutableEntry reads data from there on-demand, but the iterator needs to know about the entry and this creates a complicated interaction.

Make MutableEntry a simple data object and let the iterator iterate and populate it before returning it. Code is clearer and implementors only needs to worry about the iterator.

This fixes also MutableEntry visibility, that was preventing subclassing from out of the package.

Change-Id: I35010d1f80237e421dd51b8d3d61a8ecb03e0d01