perf python: Improve leader copying from evlist

perf python: Improve leader copying from evlist

The struct pyrf_evlist embeds the evlist requiring the copying from
things like parsed events. The copying logic handles the leader being
the event itself, but if the leader group event is a different in the
list it will cause an evsel to point to the evsel in the list that was
copied from which is bad. Fix this by adding another pass over the
evlist rewriting leaders, simplified by the introductin of two evlist
helpers.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-13-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>