Swiftlint: Rule request: private is preferred over fileprivate

[marcelofabri]

Because of Swift 3 migration, lots of declarations wouldn't need to be fileprivate.

[jpsim]

Once SwiftLint has its "compiled" mode we can even do this for internal.

[marcelofabri]

I'm closing this since #1656 was merged and it finds top level fileprivate declarations, but also has a rule to trigger on all fileprivate usage. This second one will be useful when using Swift 4, since basically it'll make possible to convert all the "acceptable" usages of fileprivate to private.