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

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### Linked PRs
* gh-154689
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Start at pathlib.Path.copy() and its ensure_distinct_paths() check, then reproduce the absolute-path and symlink examples from the issue on Linux. Done means both aliases are rejected as copying a directory into itself, while the existing relative-path behavior remains covered; review linked PR gh-154689 for work already in progress.