Restore Python install behavior from before D128230.

[stellaraccident]

Summary

In D128230, we accidentally moved the install for Python sources outside of the loop, having one install() per group of files. While it would be nice if we could do this, it means that we flatten the relative directory tree and every source ends up in the root. The right way to do this is to use FILE_SETS, which preserve the relative directory tree, but they are not available until CMake 3.23.

[stellaraccident]

FYI - another little glitch. I'm just going to land this for post-review to unblock downstreams (once the CI passes).

mlir/cmake/modules/AddMLIRPython.cmake

foreach(_src_path ${_src_paths})
       add_custom_command(
         TARGET ${name} PRE_BUILD
         COMMENT "Copying python source ${_src_path} -> ${_dest_path}"
         DEPENDS "${_src_path}"
         BYPRODUCTS "${_dest_path}"
         COMMAND "${CMAKE_COMMAND}" -E ${_link_or_copy}
             "${_src_path}" "${_dest_path}"
       )
-    endforeach()
-
       if(ARG_INSTALL_DIR)
+        # We have to install each file individually because we need to preserve
+        # the relative directory structure in the install destination.
+        # As an example, ${_source_relative_path} may be dialects/math.py
+        # which would be transformed to ${ARG_INSTALL_DIR}/dialects
+        # here. This could be moved outside of the loop and cleaned up
+        # if we used FILE_SETS (introduced in CMake 3.23).
+        get_filename_component(_install_destination "${ARG_INSTALL_DIR}/${_source_relative_path}" DIRECTORY)
         install(
-        FILES ${_src_paths}
-        DESTINATION "${ARG_INSTALL_DIR}"
+          FILES ${_src_path}
+          DESTINATION "${_install_destination}"
           COMPONENT ${ARG_INSTALL_COMPONENT}
         )
       endif()
     endforeach()
+  endforeach()
 endfunction()
 
 ################################################################################
 # Build python extension
 ################################################################################
 function(add_mlir_python_extension libname extname)
   cmake_parse_arguments(ARG
   ""