Modify evttype_index_ruleset to derive from indexable_ruleset

Modify evttype_index_ruleset to derive from indexable_ruleset instead
of having its own implementation of segregating filters by ruleset
id/event type.

An evttype_index_wrapper contains a falco rule and filter, and
implements the methods required by the template. run_wrappers()
evaluate the filter as before, without the segregation by ruleset
id/event type.

Signed-off-by: Mark Stemm <[email protected]>