perf symbols: map_groups__find_symbol must return the map too

Tools need to know from which map in the map_group a symbol was resolved
to, so that, for isntance, we can annotate kernel modules symbols by
getting its precise name, etc.

Also add the _by_name variants for completeness.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>