Fix mixed target builds when public headers path includes directories

- When constructing the VFSOverlay for the all-product-headers.yaml, the code
  did not recursively search all paths in the public headers directory.

Author: ncooke3

Sources/Basics/VFSOverlay.swift

@@ -50,9 +50,9 @@ public struct VFSOverlay: Encodable {
             case contents
         }
 
-        private let contents: [File]
+        private let contents: [Resource]
 
-        public init(name: String, contents: [File]) {
+        public init(name: String, contents: [Resource]) {
             self.contents = contents
             super.init(name: name, type: "directory")
         }
@@ -89,3 +89,44 @@ public struct VFSOverlay: Encodable {
         try JSONEncoder.makeWithDefaults(prettified: false).encode(path: path, fileSystem: fileSystem, self)
     }
 }
+
+public extension VFSOverlay {
+    /// Returns a tree of `VFSOverlay` resources for a given directory in the form of an array. Each item
+    /// in this array will be a resource (either file or directory) from the top most level of the given directory.
+    /// - Parameters:
+    ///   - directoryPath: The directory to recursively search for resources in.
+    ///   - fileSystem: The file system to search.
+    /// - Returns: An array of `VFSOverlay.Resource`s from the given directory.
+    /// - Throws: An error if the given path is a not a directory.
+    /// - Note: This API will recursively scan all subpaths of the given path.
+    static func overlayResources(
+        directoryPath: AbsolutePath,
+        fileSystem: FileSystem
+    ) throws -> [VFSOverlay.Resource] {
+        return
+            // Absolute path to each resource in the directory.
+            try fileSystem.getDirectoryContents(directoryPath).map(directoryPath.appending(component:))
+            // Map each path to a corresponding VFSOverlay, recursing for directories.
+            .compactMap { resourcePath in
+                if fileSystem.isDirectory(resourcePath) {
+                    return VFSOverlay.Directory(
+                        name: resourcePath.basename,
+                        contents:
+                            try overlayResources(
+                                directoryPath: resourcePath,
+                                fileSystem: fileSystem
+                            )
+                    )
+                } else if fileSystem.isFile(resourcePath) {
+                    return VFSOverlay.File(
+                        name: resourcePath.basename,
+                        externalContents: resourcePath.pathString
+                    )
+                } else {
+                    // This case is not expected to be reached as a resource
+                    // should be either a file or directory.
+                    return nil
+                }
+            }
+    }
+}

Sources/Build/BuildPlan.swift

@@ -1453,15 +1453,10 @@ public final class MixedTargetBuildDescription {
                     name: publicHeadersPath.pathString,
                     contents:
                         // Public headers
-                        try Set(fileSystem.getDirectoryContents(publicHeadersPath)
-                                .map(publicHeadersPath.appending(component:)))
-                                .filter(publicHeadersPath.isAncestor(of:))
-                                .map { headerPath in
-                                    VFSOverlay.File(
-                                        name: headerPath.basename,
-                                        externalContents: headerPath.pathString
-                                    )
-                                }
+                    	try VFSOverlay.overlayResources(
+                            directoryPath: publicHeadersPath,
+                            fileSystem: fileSystem
+                        )
                 ),
                 VFSOverlay.Directory(
                     name: buildArtifactDirectory.pathString,