perf jevents: Add load event JSON to verify and allow fallbacks

perf jevents: Add load event JSON to verify and allow fallbacks

Add a LoadEvents function that loads all event JSON files in a
directory.

In the Event constructor ensure all events are defined in the event JSON
except for legacy events like "cycles".

If the initial event isn't found then legacy_event1 is used, and if that
isn't found legacy_event2 is used.

This allows a single Event to have multiple event names as models will
often rename the same event over time. If the event doesn't exist an
exception is raised.

So that references to metrics can be added, add the MetricRef
class. This doesn't validate as an event name and so provides an
escape hatch for metrics to refer to each other.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Falcon <thomas.falcon@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Benjamin Gray <bgray@linux.ibm.com>
Cc: Caleb Biggers <caleb.biggers@intel.com>
Cc: Edward Baker <edward.baker@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Leo Yan <leo.yan@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Perry Taylor <perry.taylor@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Xu Yang <xu.yang_2@nxp.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>