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`.

datalad_next/iter_collections/tarfile.py

@@ -22,7 +22,11 @@
 
 @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``."""
 
 
 def iter_tar(
@@ -63,7 +67,7 @@ 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,