Make TarfileItem.name be of type PurePosixPath

Rational from #409:

```py
>>> PureWindowsPath(*PurePosixPath('d/a\\b\\c.txt').parts)
PureWindowsPath('d/a/b/c.txt')
```

This means that we must relay the POSIX nature of the archive member
path to the users, because there is no way to express this as a platform
(windows) path -- and also no way to extract this file under an
equivalent name on an FS that uses windows-semantics. So a type
mismatch can be used to trigger mitigation strategies.

If feel like a clean(er) solution would be to change `TarFileItem` to
declare to have a `name` of type `PurePosixPath`.

For the same reason and rational, a symlink target must also be
communicated in POSIX form.

datalad_next/iter_collections/tarfile.py

@@ -8,7 +8,6 @@
 from dataclasses import dataclass
 from pathlib import (
     Path,
-    PurePath,
     PurePosixPath,
 )
 import tarfile
@@ -22,7 +21,14 @@
 
 @dataclass  # sadly PY3.10+ only (kw_only=True)
 class TarfileItem(FileSystemItem):
-    pass
+    name: PurePosixPath
+    """TAR uses POSIX paths as item identifiers. Not all POSIX paths can
+    be represented on all (non-POSIX) file systems, therefore the item
+    name is represented in POSIX form, instead of a platform-dependent
+    ``PurePath``."""
+    link_target: PurePosixPath | None = None
+    """Just as for ``name``, a link target is also reported in POSIX
+    format."""
 
 
 def iter_tar(
@@ -63,14 +69,14 @@ def iter_tar(
                 else FileSystemItemType.hardlink if member.islnk() \
                 else FileSystemItemType.specialfile
             item = TarfileItem(
-                name=PurePath(PurePosixPath(member.name)),
+                name=PurePosixPath(member.name),
                 type=mtype,
                 size=member.size,
                 mode=member.mode,
                 mtime=member.mtime,
                 uid=member.uid,
                 gid=member.gid,
-                link_target=PurePath(PurePosixPath(member.linkname))
+                link_target=PurePosixPath(member.linkname)
                 if member.linkname else None,
             )
             if fp and mtype in (

datalad_next/iter_collections/tests/test_itertar.py

@@ -1,4 +1,4 @@
-from pathlib import PurePath
+from pathlib import PurePosixPath
 import pytest
 
 from datalad.api import download
@@ -47,24 +47,24 @@ def test_iter_tar(sample_tar_xz):
                    'md5': 'ba1f2511fc30423bdbb183fe33f3dd0f'}
     targets = [
         TarfileItem(
-            name=PurePath('test-archive'),
+            name=PurePosixPath('test-archive'),
             type=FileSystemItemType.directory,
             size=0,
             mtime=1683657433,
             mode=509,
             uid=1000,
             gid=1000),
         TarfileItem(
-            name=PurePath('test-archive') / '123.txt',
+            name=PurePosixPath('test-archive') / '123.txt',
             type=FileSystemItemType.symlink,
             size=0,
             mtime=1683657414,
             mode=511,
             uid=1000,
             gid=1000,
-            link_target=PurePath('subdir') / 'onetwothree_again.txt'),
+            link_target=PurePosixPath('subdir') / 'onetwothree_again.txt'),
         TarfileItem(
-            name=PurePath('test-archive') / '123_hard.txt',
+            name=PurePosixPath('test-archive') / '123_hard.txt',
             type=FileSystemItemType.file,
             size=4,
             mtime=1683657364,
@@ -73,15 +73,15 @@ def test_iter_tar(sample_tar_xz):
             gid=1000,
             link_target=None),
         TarfileItem(
-            name=PurePath('test-archive') / 'subdir',
+            name=PurePosixPath('test-archive') / 'subdir',
             type=FileSystemItemType.directory,
             size=0,
             mtime=1683657400,
             mode=509,
             uid=1000,
             gid=1000),
         TarfileItem(
-            name=PurePath('test-archive') / 'subdir' / 'onetwothree_again.txt',
+            name=PurePosixPath('test-archive') / 'subdir' / 'onetwothree_again.txt',
             type=FileSystemItemType.file,
             size=4,
             mtime=1683657400,
@@ -90,14 +90,14 @@ def test_iter_tar(sample_tar_xz):
             gid=1000,
             link_target=None),
         TarfileItem(
-            name=PurePath('test-archive') / 'onetwothree.txt',
+            name=PurePosixPath('test-archive') / 'onetwothree.txt',
             type=FileSystemItemType.hardlink,
             size=0,
             mtime=1683657364,
             mode=436,
             uid=1000,
             gid=1000,
-            link_target=PurePath('test-archive') / '123_hard.txt'),
+            link_target=PurePosixPath('test-archive') / '123_hard.txt'),
     ]
     ires = []
     for i in iter_tar(sample_tar_xz, fp=True):