[Swift APIView] Native types do not appear in the navigation pane when they have extensions

[tjprescott]

When a package defines an extension for a Swift Native type, that type does not appear in APIView's navigation.

public extension Double {
    var km: Double { return self * 1_000.0 }
    var m: Double { return self }
    var cm: Double { return self / 100.0 }
    var mm: Double { return self / 1_000.0 }
    var ft: Double { return self / 3.28084 }
}

While this extension does appear in the APIView, it does not appear in the navigation panel. It should probably live alongside the package-defined types.

Swift
 |--> Double
SwiftAPIViewTests
 |--> SomeClass