Validate explicit ACL modifiers for top-level declarations

[jpsim]

Top-level declarations should use explicit ACL keywords (public, internal, private).

[keith]

It looks like there's no difference to sourcekit whether or not internal is specified. Using sourcekitten's structure the only difference between:

class Foo {}

And:

internal class Foo {}

Is the offset. This would have to be handled by examining the file as well.

[segiddins]

@keith examining the contents of the declaration would probably suffice for this?

[keith]

It seems like the class declaration key's offset starts after the modifiers. So in the second example from above the offset is 9. This could cause issue for cases like:

internal final class Foo {}

Or something crazier like:

internal
class Foo {}

[jpsim]

Here's a rough algorithm to do this:

1. find all top-level internal type declarations: see https://github.com/realm/SwiftLint/blob/master/Source/SwiftLintFramework/Rules/TypeNameRule.swift.
2. find all internal modifiers: File.matchPattern("internal", withSyntaxKinds: [.AttributeBuiltin]).
3. throw style violations for declarations found in step 1 with no associated internal modifier from step 2.

[jdmarshall90]

Could this be enhanced to include all declarations, and not just top-level ones?