GH-139174: Prepare `pathlib.Path.info` for new methods (part 2) (#140155)

Merge `_Info`, `_StatResultInfo` and `_DirEntryInfo` into a single `_Info`
class. No other changes.

This will allow us to use a cached `os.stat()` result from our upcoming
`_Info.stat()` method even when we have a backing `os.DirEntry`.