GH-110109: Churn `pathlib.PurePath` methods (#112012)

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

The `PurePath` implementations of certain special methods, such as
`__eq__()` and `__hash__()`, are not usually applicable to user subclasses
of `_PathBase`. To facilitate their removal, another patch will split the
`PurePath` class into `_PurePathBase` and `PurePath`, with the latter
providing these special methods.

This patch prepares the ground for splitting `PurePath`. It's similar to
e8d77b0, which preceded splitting `Path`. By churning the methods here,
subsequent patches will be easier to review and less likely to break
things.