to_disk: Represent revisions as empty directories instead of symlinks

The symlink representation is slightly ambiguous, so not necessarily
very useful.
Additionally, it made directory-flat bundles differ from a non-recursive
'git clone' checkout, which confuses Guix and potentially other tools.

We may revisit this in the future, eg. by adding a dotfile in the
directory or metadata at the root.

swh/vault/tests/test_cookers.py

@@ -580,8 +580,8 @@ class TestDirectoryCooker:
         with cook_extract_directory_dircooker(
             swh_storage, dir.swhid(), fsck=False
         ) as p:
-            assert (p / "submodule").is_symlink()
-            assert os.readlink(str(p / "submodule")) == target_rev
+            assert (p / "submodule").is_dir()
+            assert list((p / "submodule").iterdir()) == []
 
 
 class RepoFixtures:

swh/vault/to_disk.py

@@ -133,8 +133,7 @@ class DirectoryBuilder:
         identifier."""
         for file_data in revs_data:
             path = os.path.join(self.root, file_data["path"])
-            target = hashutil.hash_to_bytehex(file_data["target"])
-            self._create_file(path, target, mode=DentryPerms.symlink)
+            os.makedirs(path)
 
     def _create_file(
         self, path: bytes, content: bytes, mode: int = DentryPerms.content