Improve performance of `os.walk()`

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
• gh-121431
• gh-121432
• gh-121433
• gh-121435

Start at the os.walk() entry point and inspect its use of os.scandir(), entry.is_dir(), entry.is_symlink(), and os.path.join(). Review linked PRs gh-119186, gh-119473, gh-119573, gh-119750, gh-121431, gh-121432, gh-121433, and gh-121435 before making changes; done means the proposed performance improvements preserve exception handling and traversal behavior.