Nested structure for Kibana App plugins

[timroes]

Since #68811 we can now group plugins together into folders. I think we should discuss how we want to group the Kibana App plugins into folders, since we're having quiet a bunch of them. I'd put up the following structure up for discussion and would value any input:

• plugins
  • discover (leave at the root level for now)
  • dashboard (leave that at the root level for now)
  • timelion (the application, leave this since it's scheduled for removal in 8.0)
  • management
    • management_app (owned by App Arch)
    • advanced_settings
    • saved_objects_management (owned by Platform)
  • visualizations
    • visualize (the application)
    • visualizations (the infrastructure, planned to be merged together in long run with visualize)
    • vis_default_editor (rename to default_editor potentially, since now nested)
    • vis_type_*
    • region_map
    • tile_map
• x-pack/plugins (leave everything here on top level, since there's currently not really any more reasonable grouping)
  • discover_enhanced
  • dashboard_enhanced
  • graph
  • lens
  • dashboard_mode (scheduled for removal in 8.0)

cc @elastic/kibana-app-arch and @elastic/kibana-platform since this makes some assumptions about your plugins too.

[pgayvallet]

Moving saved_objects_management under a management folder/section SGTM

[lukeelmers]

@timroes Just as a heads up, Josh recently made a wider proposal for updating the repo directory structure in #71566

[timroes]

Closing this in favor of #71566