Add Swift Package Manager support

Add Package.swift with three products: GCDWebServer (core),
GCDWebUploader, and GCDWebDAVServer.

Public headers live across Core/Requests/Responses subdirectories, which
SPM cannot expose directly. To support SPM without moving any files
(keeping the Xcode project and CocoaPods builds intact), public headers
are surfaced through symlinks in include/ directories with a hand-written
umbrella-directory module map; the private GCDWebServerPrivate.h is left
unlinked so it stays out of the public module.

GCDWebUploader.bundle is shipped as a .copy resource and located at
runtime via SWIFTPM_MODULE_BUNDLE under a #if SWIFT_PACKAGE branch.

Verified: swift build (all targets), Swift module imports, Xcode
framework build, and the XCTest suite all pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

GCDWebUploader/GCDWebUploader.m

@@ -66,7 +66,12 @@ NS_ASSUME_NONNULL_END
 
 - (instancetype)initWithUploadDirectory:(NSString*)path {
   if ((self = [super init])) {
+#if SWIFT_PACKAGE
+    // When built with Swift Package Manager, resources live inside the auto-generated module bundle
+    NSString* bundlePath = [SWIFTPM_MODULE_BUNDLE pathForResource:@"GCDWebUploader" ofType:@"bundle"];
+#else
     NSString* bundlePath = [[NSBundle bundleForClass:[GCDWebUploader class]] pathForResource:@"GCDWebUploader" ofType:@"bundle"];
+#endif
     if (bundlePath == nil) {
       return nil;
     }