Re-arrange `pathlib.Path` methods in source code. No other changes.

The methods are arranged as follows:

1. `stat()` and dependants (`exists()`, `is_dir()`, etc)
2. `open()` and dependants (`read_text()`, `write_bytes()`, etc)
3. `iterdir()` and dependants (`glob()`, `walk()`, etc)
4. All other `Path` methods

This patch prepares the ground for a new `_AbstractPath` class, which will
support the methods in groups 1, 2 and 3 above. By churning the methods
here, subsequent patches will be easier to review and less likely to break
things.