objectstore: re-use Object.export in Object.init

Instead of having a duplication of the invocation of `cp`, once in
`init`, once in `export`, re-use the latter in the former: the to
be copied object is accessed in the normal way via the store, and
then "exported" to the new location. This gets rid of the call to
resolve_ref as a nice side effect, which means less poking into
the internals of the store.

osbuild/objectstore.py

@@ -59,10 +59,8 @@ class Object:
         if self._init:
             return
 
-        source = self.store.resolve_ref(self._base)
+        with self.store.new(self._base) as obj:
-        subprocess.run(["cp", "--reflink=auto", "-a",
+            obj.export(self._tree)
-                        f"{source}/.", self._tree],
-                       check=True)
         self._init = True
 
     @property