[5.8] Prevent event cache from firing multiple times the same event(s)

[jf-m]

This is a proposed solution for solving #28872 .

Changes explanations:

EventCacheCommand

• The discoverEvents are cached only when shouldDiscoverEvents is true on the related providers, otherwise only the listens are cached.
• I've changed the structure of events.php so the the events/Listeners are grouped by EventServiceProvider.

[
  [Arrayof EventServiceProviders] => [Arrayof Events] => [Arrayof Listeners] 
]

instead of the old structure :

[
   [Arrayof Events] => [Arrayof Listeners] 
]

• I've changed array_merge to array_merge_recursive because it still is technically possible that a basic listen event is also inside a discovered event.

EventServiceProvider

• The boot methods now calls a getEvents to get the events of both discover and listen
• If the events are cached, the getEvents will return ONLY the events located under the name of the current EventServiceProvider in the events.php cache.
  Indeed, the $this->listens() are already cached and they are all located inside the cache file, so there is no need to merge the cache file with listens;
  There is also no need to check for shouldDiscoverEvents because it was already done inside the event:cache command.

I hope this helps, cheers !