Don't attempt to read directories with a .swift path extension as files

Fixes #13

Author: mattt

Sources/swift-doc/main.swift

@@ -28,6 +28,7 @@ command(
                 let directory = URL(fileURLWithPath: path)
                 guard let directoryEnumerator = fileManager.enumerator(at: directory, includingPropertiesForKeys: nil) else { continue }
                 for case let url as URL in directoryEnumerator {
+                    guard !url.hasDirectoryPath else { continue }
                     guard url.pathExtension == "swift" else { continue }
                     sourceFiles.append(try SourceFile(file: url, relativeTo: directory))
                 }