Replace Prefire with a very similar but simpler and more direct approach (#3232)

* Replace Prefire with a very similar but simpler and more direct approach

* Move PreviewTest snapshots to their new location (snapshotting default, mirrors UITests)

* Update acknowledgements

* Switch snapshot configuration extensions to a custom version

* Exclude snapshots from the PreviewTests target

* Fix acknowledgements

* Rename snapshotting preference keys

* Remove `skipPackagePluginValidation`

fastlane/Fastfile

@@ -95,7 +95,6 @@ lane :unit_tests do |options|
       ensure_devices_found: true,
       result_bundle: true,
       number_of_retries: 3,
-      xcargs: '-skipPackagePluginValidation',
     )
   end