Improve search paths for XCFrameworks

When packaging a Swift dylib as an XCFramework, its modules will be at the top-level, so that directory needs to become part of search paths.

fixes #5723

Author: neonichu

Sources/Build/BuildPlan.swift

@@ -2130,8 +2130,8 @@ public class BuildPlan {
                 if case .xcframework = target.kind {
                     let libraries = try self.parseXCFramework(for: target)
                     for library in libraries {
-                        if let headersPath = library.headersPath {
-                            clangTarget.additionalFlags += ["-I", headersPath.pathString]
+                        library.headersPaths.forEach {
+                            clangTarget.additionalFlags += ["-I", $0.pathString]
                         }
                         clangTarget.libraryBinaryPaths.insert(library.libraryPath)
                     }
@@ -2167,8 +2167,8 @@ public class BuildPlan {
                 if case .xcframework = target.kind {
                     let libraries = try self.parseXCFramework(for: target)
                     for library in libraries {
-                        if let headersPath = library.headersPath {
-                            swiftTarget.additionalFlags += ["-Xcc", "-I", "-Xcc", headersPath.pathString]
+                        library.headersPaths.forEach {
+                            swiftTarget.additionalFlags += ["-I", $0.pathString, "-Xcc", "-I", "-Xcc", $0.pathString]
                         }
                         swiftTarget.libraryBinaryPaths.insert(library.libraryPath)
                     }

Sources/SPMBuildCore/BinaryTarget+Extensions.swift

@@ -23,8 +23,8 @@ public struct LibraryInfo: Equatable {
     /// The path to the binary.
     public let libraryPath: AbsolutePath
 
-    /// The path to the headers directory, if one exists.
-    public let headersPath: AbsolutePath?
+    /// The paths to the headers directories.
+    public let headersPaths: [AbsolutePath]
 }
 
 
@@ -54,8 +54,8 @@ extension BinaryTarget {
         // Construct a LibraryInfo for the library.
         let libraryDir = self.artifactPath.appending(component: library.libraryIdentifier)
         let libraryFile = AbsolutePath(library.libraryPath, relativeTo: libraryDir)
-        let headersDir = library.headersPath.map { AbsolutePath($0, relativeTo: libraryDir) }
-        return [LibraryInfo(libraryPath: libraryFile, headersPath: headersDir)]
+        let headersDirs = library.headersPath.map({ [AbsolutePath($0, relativeTo: libraryDir)] }) ?? [] + [libraryDir]
+        return [LibraryInfo(libraryPath: libraryFile, headersPaths: headersDirs)]
     }
 
     public func parseArtifactArchives(for triple: Triple, fileSystem: FileSystem) throws -> [ExecutableInfo] {