Move various intermediate files (such as %{pyproject_files}) into %buildsubdir
    
    Apparently, when you repeatedly run `rpmbuild -ba`, files in %_builddir are not cleaned.
    This way, we at least make sure the files are located within the appropriate build subdirectory,
    so 2 unrelated builds don't share the files between each other.
    
    Keeping files contained in the build subdirectory is the common way of doing things anyway.
    
    Should be backwards compatible, as we only modify underscore-prefixed macros and %{pyproject_files},
    where the exact value should not matter to the packagers.