GH-125413: Add private `pathlib.Path` method to write metadata (#130238)

GH-125413: Add private `pathlib.Path` method to write metadata (#130238)

Replace `WritablePath._copy_writer` with a new `_write_info()` method. This
method allows the target of a `copy()` to preserve metadata.

Replace `pathlib._os.CopyWriter` and `LocalCopyWriter` classes with new
`copy_file()` and `copy_info()` functions. The `copy_file()` function uses
`source_path.info` wherever possible to save on `stat()`s.