Allow directories for external files

WebsiteGenerator/Content/Element.swift

@@ -372,7 +372,10 @@ extension Element {
     }
 
     static func rootPaths(for input: Set<String>?, path: String) -> Set<String> {
-        input.unwrapped { Set($0.map { relativeToRoot(filePath: $0, folder: path) }) } ?? []
+        guard let input = input else {
+            return []
+        }
+        return Set(input.map { relativeToRoot(filePath: $0, folder: path) })
     }
 
     func relativePathToFileWithPath(_ filePath: String) -> String {

WebsiteGenerator/Files/FileSystem.swift

@@ -386,7 +386,7 @@ final class FileSystem {
             let sourceUrl = input.appendingPathComponent(cleanPath)
             let destinationUrl = output.appendingPathComponent(cleanPath)
             guard sourceUrl.exists else {
-                if !externalFiles.contains(file) {
+                if !isExternal(file: file) {
                     log.add(error: "Missing required file", source: cleanPath)
                 }
                 continue
@@ -403,7 +403,7 @@ final class FileSystem {
             }
         }
         for (file, source) in expectedFiles {
-            guard !externalFiles.contains(file) else {
+            guard !isExternal(file: file) else {
                 continue
             }
             let cleanPath = cleanRelativeURL(file)
@@ -438,6 +438,42 @@ final class FileSystem {
         return result.joined(separator: "/")
     }
 
+    /**
+     Check if a file is marked as external.
+
+     Also checks for sub-paths of the file, e.g if the folder `docs` is marked as external,
+     then files like `docs/index.html` are also found to be external.
+     - Note: All paths are either relative to root (no leading slash) or absolute paths of the domain (leading slash)
+     */
+    func isExternal(file: String) -> Bool {
+        // Deconstruct file path
+        var path = ""
+        for part in file.components(separatedBy: "/") {
+            guard part != "" else {
+                continue
+            }
+            if path == "" {
+                path = part
+            } else {
+                path += "/" + part
+            }
+            if externalFiles.contains(path) {
+                return true
+            }
+        }
+        return false
+    }
+
+    func printExternalFiles() {
+        guard !externalFiles.isEmpty else {
+            return
+        }
+        print("\(externalFiles.count) external files needed:")
+        for file in externalFiles.sorted() {
+            print("  " + file)
+        }
+    }
+
     // MARK: Pages
 
     func isEmpty(page: String) {