iOS: Only look for packages in the provided CMAKE_FIND_ROOT_PATHs

Both CMAKE_FIND_ROOT_PATH_MODE_INCLUDE and
CMAKE_FIND_ROOT_PATH_MODE_LIBRARY are set to "ONLY" when cross
building to iOS, but appears that CMAKE_FIND_ROOT_PATH_MODE_PACKAGE
was overlooked.

This causes packages to be searched for in the host system as well,
which is incorrect and can lead to linking issues.

Set CMAKE_FIND_ROOT_PATH_MODE_PACKAGE to "ONLY" as well.

CMAKE_FIND_ROOT_PATH_MODE_PROGRAM is not touched, because a user
might want to find programs / tools on the host system.

Modules/Platform/Darwin.cmake

@@ -8,6 +8,7 @@ if(CMAKE_SYSTEM_NAME STREQUAL "iOS" OR CMAKE_SYSTEM_NAME STREQUAL "tvOS" OR CMAK
   list(APPEND CMAKE_FIND_ROOT_PATH "${_CMAKE_OSX_SYSROOT_PATH}")
   set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
   set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
+  set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
 endif()
 
 # Darwin versions: