Improve performance of `os.walk()`

[barneygale]

There are a couple of minor performance improvements possible in os.walk():

• We don't need to manually pump the os.scandir iterator, given we handle exceptions from next() like exceptions from scandir() itself, i.e. by ignoring the problematic directory and moving on. We can use a for loop like filthy casuals.
• ~In bottom-up mode, we can handle exceptions from entry.is_symlink() in the same block as those from entry.is_dir(), which avoids a few temporary variables.~
• In top-down mode, we can call os.path.join() once on a parent directory rather than for each child path.

Linked PRs

• gh-119186
• gh-119473
• gh-119573
• gh-119750

[barneygale]

In bottom-up mode, we can handle exceptions from entry.is_symlink() in the same block as those from entry.is_dir(), which avoids a few temporary variables.

Upon closer inspection, I think I was wrong. Symlinks to directories where is_symlink() raises should still be categorised as dirs.