Restore Python install behavior from before D128230.

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.

Differential Revision: https://reviews.llvm.org/D129434