gdb/dwarf: rename cooked_index_worker subclasses

Rename them to include "worker" in the name.  Otherwise, it's easy to be
confused and think that they are sub-classes of "cooked_index".

Change-Id: I625ef076f9485f3873db530493f60a53d02c1991
Approved-By: Tom Tromey <tom@tromey.com>