xcode: Override determine_swift_deps_dir()

Xcode does not recognize our private directories, nor does it ever
try to generate them. Instead, just import the build directories
for each swift dependency.

This fixes linking between swift targets when they are in their own
subdirectories, as they will have different build directories in that
case.

mesonbuild/backend/xcodebackend.py

@@ -255,6 +255,13 @@ class XCodeBackend(backends.Backend):
         obj_path = f'{project}.build/{buildtype}/{tname}.build/Objects-normal/{arch}/{stem}.o'
         return obj_path
 
+    def determine_swift_dep_dirs(self, target):
+        result: T.List[str] = []
+        for l in target.link_targets:
+            # Xcode does not recognize our private directories, so we have to use its build directories instead.
+            result.append(os.path.join(self.environment.get_build_dir(), self.get_target_dir(l)))
+        return result
+
     def generate(self, capture: bool = False, vslite_ctx: dict = None) -> T.Optional[dict]:
         # Check for (currently) unexpected capture arg use cases -
         if capture: