pathlib.Path.copy() can copy a directory into itself via absolute paths or symlinks

Bug report

Bug description:

Bug report

Path.copy() guards against copying a directory into itself with ensure_distinct_paths(), but the check is lexical only. Relative vs absolute paths and symlink aliases bypass it.

from pathlib import Path
import os

Path('src').mkdir()
(Path('src') / 'f').write_text('hello')

Path('src').copy('src/nested')  # OSError - OK

Path('src').copy(Path('src').resolve() / 'nested')  # succeeds - copies into itself

os.symlink('src', 'link')
Path('src').copy('link/nested')  # also succeeds

Same class of bug as gh-149835 that was fixed with `realpath`, but `pathlib` still uses a lexical comparison.

### CPython versions tested on:

CPython main branch

### Operating systems tested on:

Linux

<!-- gh-linked-prs -->
### Linked PRs
* gh-154689
<!-- /gh-linked-prs -->

从 pathlib.Path.copy() 及其 ensure_distinct_paths() 检查入手，然后在 Linux 上复现 issue 中的绝对路径和 symlink 示例。完成的标准是：两个别名都会因试图将目录复制到其自身中而被拒绝，同时现有的相对路径行为仍得到覆盖；查看链接的 PR gh-154689，了解已经在进行的工作。