[MLIR][Python] Use DEST_PREFIX when installing.

mlir/cmake/modules/AddMLIRPython.cmake

function(add_mlir_python_modules name)
   # Helper to process an individual target.
   function(_process_target modules_target sources_target)
     get_target_property(_source_type ${sources_target} PYTHON_SOURCES_TYPE)
     if(_source_type STREQUAL "pure")
       # Pure python sources to link into the tree.
       get_target_property(_python_root_dir ${sources_target} PYTHON_ROOT_DIR)
       get_target_property(_python_sources ${sources_target} PYTHON_SOURCES)
       get_target_property(_specified_dest_prefix ${sources_target} PYTHON_DEST_PREFIX)
-      set(_dest_prefix "${ARG_ROOT_PREFIX}")
+      foreach(_source_relative_path ${_python_sources})
+        set(_dest_relative_path "${_source_relative_path}")
         if(_specified_dest_prefix)
-        set(_dest_prefix "${_dest_prefix}/${_specified_dest_prefix}")
+          set(_dest_relative_path "${_specified_dest_prefix}/${_dest_relative_path}")
         endif()
-      foreach(_source_relative_path ${_python_sources})
         set(_src_path "${_python_root_dir}/${_source_relative_path}")
-        set(_dest_path "${_dest_prefix}/${_source_relative_path}")
+        set(_dest_path "${ARG_ROOT_PREFIX}/${_dest_relative_path}")
 
         get_filename_component(_dest_dir "${_dest_path}" DIRECTORY)
-        get_filename_component(_install_path "${ARG_INSTALL_PREFIX}/${_source_relative_path}" DIRECTORY)
+        get_filename_component(_install_path "${ARG_INSTALL_PREFIX}/${_dest_relative_path}" DIRECTORY)
 
         file(MAKE_DIRECTORY "${_dest_dir}")
         add_custom_command(
           TARGET ${modules_target} PRE_BUILD
           COMMENT "Copying python source ${_src_path} -> ${_dest_path}"
           DEPENDS "${_src_path}"
           BYPRODUCTS "${_dest_path}"
           COMMAND "${CMAKE_COMMAND}" -E create_symlink