perf maps: Introduce maps__find_symbol_by_name()

perf maps: Introduce maps__find_symbol_by_name()

Out of map_groups__find_symbol_by_name(), so that we can turn this later
one first into a call to maps__find_symbol_by_name(MAP__FUNCTION) +
MAP__VARIABLE, and then to just one call, we'll merge MAP__FUNCTION with
MAP__VARIABLE maps, to simplify the code.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-pvkar0jacqn92g148u9sqttt@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>