codesearchidx: use {topdir} for consistency

codesearchidx: use {topdir} for consistency

There's no reason for the read-only CodeSearch and read-write
CodeSearchIdx to use different field names for referring to the
same thing.  Consistently use {topdir} since it's inspired by
ExtSearch{,Idx} and also matches the configuration knob.  This
will make sharing methods between CodeSearch and CodeSearchIdx
easier.