[Rule request] Internal as private

[kimdv]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

New rule request

Please describe the rule idea, format
this issue's title as Rule Request: [Rule Name] and describe:
When a method is internal but not used outside of class the rule should warn to make it private.

Provide several examples of what would and wouldn't trigger violations.

Nontrigger

 """
class Foo {
    func bar() { }
}

let foo = Foo()
foo.bar()
"""

Trigger

 """
class Foo {
    func bar() { }
    private func baz() {
        bar()
    }
}

"""

Should the rule be opt-in or enabled by default? Why?
Because it is analyzer rule it should be opt-in.

[jpsim]

I think this will require multifile support.

[jorgealegre]

I would love to have this.