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

从 os.walk() 入口点开始，检查其对 os.scandir()、entry.is_dir()、entry.is_symlink() 和 os.path.join() 的使用。在进行更改之前，先审查关联的 PR：gh-119186、gh-119473、gh-119573、gh-119750、gh-121431、gh-121432、gh-121433 和 gh-121435；当所提议的性能改进保留异常处理和遍历行为时，即表示完成。